Theme: Something you find on a fish. No, not FINS or GILLS. Think more musically - no, not even Vince Gill - and the reveal will make it clear. So let's start there.
38 A. Musical sequence found at the starts of the answers to the starred clues : SCALE. This word has many disparate meanings as both a noun and a verb. Here, we are talking about a set of musical notes, ordered by pitch. This puzzle has them ordered in ascending sequence, though, of course, you can also go the other way. We're not told if the quality of the scale is major, minor, nor if it is one of the many modal variants. But we can sing along anyway.
The beginnings of the first words of eight in-the-language phrases name the notes of the SCALE.
1 A. *Where deliveries may be left : DOOR STEP. The spot where mail-order deliveries end up. DO is the first scale note, and the tonal center, aka tonic, for the SCALE and any music based upon it.
17 A. *Airport pickup : RENT-A-CAR. The plane does not take you to your final destination. You usually need to drive there, and you can use a conveniently located vehicle for a price. RE is the 2nd SCALE step, a whole tone above the tonic. We won't talk about the Phrygian mode, the comma of Pythagorus, nor vibrational frequencies.
20 A. *Modest garment : MIDI-SKIRT. Mid-calf-length to protect delicate legs from winter winds and prying eyes. Seems a bit awkward. MI is the third scale step, another whole tone above RE in a major scale, but only a half tone up if the quality of the SCALE is minor. BTW - the distance from one note to another is called the interval.
26 A. *Hiker's pouch : FANNY PACK. A container on a belt, often perched over a person's back side. [But not in England.] FA is the fourth scale step above the tonic. This note does not vary between major and minor scales, and its interval above the tonic is called the perfect fourth.
51 A. *Infant's dietary prohibition : SOLID FOOD. Mom's milk is best, formula is OK. Save the bran flakes for when they're a bit older. SOL is the fifth scale step, always a perfect fifth above the tonic.
58 A. *Breaking point : LAST STRAW. An ancient measure of camel capacity being the last straw minus one. LA is the 6th scale step, a major or minor sixth above the tonic, depending on the quality of the scale.
67 A. *History book chart : TIME LINE. A graphic representation of what happened when. TI is the penultimate SCALE step, either a major or minor seventh interval above the tonic. As you may have gleaned by now, the natural minor SCALE differs from the major by having lowered 3rd, 6th, and 7th SCALE steps. There are other versions, but we needn't get into that here
72 A. *Words of admonishment : DON’T DO IT. Have we heard these words? Have we heeded them? Probably not. This brings us to another DO, a perfect octave above where we started. Despite what I said earlier, I'll point out that the frequency is exactly twice that of the previous DO.
Hi gang. JazzBumpa conducting for today's thematically rich excursion. Note that though the spellings are right on for the eight notes, the pronunciations vary a bit. Rogers and Hammerstein wrote a song, based on a major scale, that illustrates all this, but I hate it and will not link. However, I will wave my arms around - so let's see where the music takes us.
Across
9. Adorn in relief : EMBOSS. To carve, mold or stamp a design into a surface.
15. First three of ten digits : AREA CODE. So you can phone home.
16. Bad guy : MEANIE. Like Kylo Ren.
18. State with conviction : ASSERT. Or aver or avow.
19. "It's __ cause" : A LOST. Words of giving up.
22. Extra-play qtrs. : OTs. Over Times.
24. Influenced by, in recipes : A LA. In the style of whoever the influence might be.
25. July baby, maybe : LEO. Astrology sign usually associated with August, actually running from July 23 to Aug 22. LEOS are generous, loyal and supportive.
31. Storm winds : GALES.
33. Have yet to pay : OWE.
34. NFC South team : ATL. The ATLanta Falcons of the National Football Conference South, a division of the National Football League.
35. Part of rpm : PER.
36. Whipped cream amount : GLOB. Non-specific quantity, but I'll take two, thank you.
40. Authoritarian figure : CZAR. Named for Russian despots of a by-gone era.
44. Chair part : ARM. What - no love for the leg? Is that why we have the MIDI SKIRT?
46. Balderdash : ROT. Nonsense.
47. Poetic tribute : ODE.
48. Volunteer's offer : I'LL GO.
54. Buck's mate : DOE. A deer, my dear, and a thematic homophone.
55. Sun Devils of the Pac-12 : Arizona State Univesity.
57. Afternoon social : TEA. In which the beverage is accompanied by sandwiches or cakes.
62. Giant : TITAN. From Greek mythology, the twelve TITANS were gigantic second order deities, six female and six male. The familiar Greek pantheon of Zeus, Hades, Poseidon, Hestia, Hera and Demeter were the offspring of the Titan Cronus.
66. Where it originally was : IN SITU. From Latin for "in place."
69. Had to have : NEEDED.
70. Cordial with a licorice-like flavor : ANISETTE. Last week we learned that Anise, the flavor element here, is an herb in the carrot family. So why is there no cordial with a carrot-like flavor?
71. Prone to avarice : GREEDY.
Down
2. Pitcher Hershiser : OREL. He played in the major leagues for 18 years with 5 different teams, won the Cy Young Award, was a 3-time all star, and World Series MVP. Not bad.
3. Wine lover's prefix : OENO. Into English from ancient Greek.
4. Exposes to the cops : RATS ON. Alternatively, sings like a birdie.
5. Absent-minded, to a Brit : SCATTY. From scatter-brained, I assume.
6. Front-of-bk. list : TOC. Table Of Contents.
7. Red-coated cheese : EDAM. From the eponymous town in The Netherlands.
8. Prefix with scope : PERI-. The submarines monocular.
9. Text alternative : E-MAIL. Two forms of communication by device.
10. Many a text : MESSAGE. I'm trying to imagine a text that is not a MESSAGE, and coming up short.
11. Get some rays : BASK. In the sun - for pleasure and relaxation. Don't get burned.
12. "The Hairy Ape" playwright : O'NEILL. Eugene. The play is about class struggle and alienation.
13. Yes or no follower : SIRREE. Colloquial emphasis.
14. Brief arguments : SET TOs. Or hockey fights - which also generally do not last long.
21. Divided terr. : DAKota. North and South.
23. Cramp, say : SPASM. Involuntary muscle contraction.
26. Weather condition in the final scene of "Casablanca" : FOG. But it's the start of a beautiful friendship
27. Belt maker's tool : AWL. A pointy tool for poking holes.
28. Opposite of paleo- : NEO. Modernizing a blast from the past. Paleo- indicates adhering to old ways as they always were. Can't quite see these as opposites.
29. Flight coordinators: Abbr. : ATC. Air Traffic Controllers.
30. Light cigar wrapper : CLARO. The wrapper is the outer leaf of a cigar, and can influence its characteristics. CLARO is yellowish or light tan and usually indicates a shade grown tobacco.
32. Part of a ring : ARC. A ring is shaped like a circle and a circle can be divided into arcs - but does anyone divide a ring this way? Hmmmm.
35. __ point: embroidery stitch : PETIT. A very small, fine stitch that allows detail in the design of the article.
37. Groceries quantity : BAG. Paper or plastic?
39. "u r a riot!" : LOL. Laughing Out Loud. Texter's appreciations for something funny.
41. Menagerie : ZOO. An exhibit of captive wild animals.
42. Excitement : ADO. Trouble, difficulty or fuss.
43. Like the bull in the Chicago Bulls' logo : RED.
48. In neutral : IDLING. Running, but out of gear.
49. Temporary wheels : LOANER.
50. Tenant : LESSEE. Renter with a lease agreement.
51. California's Big __ : SUR. Approximately 90 miles of rugged coastline from the Carmel River in Monterey south to San Carpoforo Creek in San Luis Obispo County. The name is derived from Spanish for "The Big South" relative to the city of Monterey.
52. Can't stand : DETEST.
53. Didn't hit the books? : FAILED. Neglected to prepare for a test in school.
56. Hit the books : STUDY. Prepare for a test in school, and back to back clehos.
59. It has its highs and lows : TIDE. Ocean levels.
60. Minimally : A TAD. Possibly derived from tadpole, the larval stage of a frog, transferred thence to a small child, and than representing a small quantity.
61. Skid row regular : WINO. Wine-addicted alcoholic.
64. Voting no : ANTI.
65. Discontinued depilatory : NEET. Evidently the name was discontinued, but the product wasn't. It is now sold under the trade name VEET.