246 ARCADiE. 



and vol. ii. p. 44. — Sowerby, Concholog. Manual, f. 134. 

 — Hanl. Recent Shells, vol. i. p. 162, pi. 10, Area, 

 f. 36, 37. 

 Area minima, Turt. Conch. Diction, p. 8. 

 Pectunculus undatus, Turt. Dithyra Brit. p. 173, pi. 12, f. 3, 4. 



„ decussaius, TvRT. Dithyra Brit. p. 173, pi. 12, f. 5. — Fleming, 



Brit. Anim. p. 400. — Brit. Marine Conch, p. 104. 

 „ nummariiis, Turt. Dithyra Brit. p. 174, pi. 12, f. 6. — Fleming, 



Brit. Anim. p. 400. — Brit. Marine Conch, p. 104. 

 „ variabilis (fossil), Sowerby, Min. Conch, t. 471, f. 1. 



Encyclopedie Methodique, Vers, pi. 310, f. 3. 



Retaining in preference the name glycimeris^ an epithet 

 bestowed upon this shell even prior to the days of Liunteus, 

 in whose "Sjstema'''' it appeared in the tenth edition, 

 pilosus being first annexed in the twelfth, we have regarded 

 the two shells as forming but one species, inasmuch as 

 those features upon whose existence their separation has 

 been based, to wit, the obliquity of contour and greater 

 inequality of the sides in the former, its angularity on the 

 more produced side, and the radiating instead of concentric 

 style of painting, are not constantly and collectively imited 

 in each individual, but so commingled in different exam- 

 ples that the exact line of demarcation cannot possibly 

 be determined. Nevertheless, a sufficient preponderance 

 of the differential characters is usually present in each 

 shell, to enable us to divide the species into two well- 

 marked and easily distinguishable varieties. 



Both these have the ordinary orbicular form of the 

 PectuncuU, the breadth of the valves being more frequently 

 (not invariably) superior in the former variety, their length 

 in the latter. In convexity both vary from scarcely ven- 

 tricose to actually tumid, the profundity being chiefly um- 

 bonal, and rather rapidly diminishing at the sides and base. 

 The structure is solid, opaque, and often ponderous ; the 

 exterior is never lustrous, but either dull or but shghtly 



