318 TESTACEOUS MOLLUSCA. 



Smooth, rather thin ; apex distant from the margin ; disk of ad- 

 hesion oblong, not close to the dorsal margin ; perforation short, 

 situated near the dorsal end of the disk of adhesion, f . Chili. 



D. (0.) Tenuis. Sow. Th. 1, p. 266, t. 73,/. 4, 5. Smooth, thin, 

 apex of the upper valve near the dorsal margin ; disk of adhesion 

 obtusely triangular, close to the dorsal edge ; perforation linear, f . 



D. (6.) Cumingii. Brod. Z. P. 1833, p. 124 ; Z. T. 1, t. 28,/. 1. 

 — Desk. ed. Lam. l,p. Sll.—Soiv. Th. I, p. 366, t, 73,/. 6. Nearly 

 orbicular, subcorneal, rather thick, with numerous stride radiating 

 from the subcentral apex and decussated by the elevated lines of 

 growth ; epidermis pale brown, £. Mazatlan, near Panama. 



D. (0.) Striata. — Discina Ostreoides, Lam. (no definition). — 

 O. Norwegica (not Lam.), Sow. Tr. Lin. 13, pi. 26,/. 2. — Soiv. G. — 

 Peeve, Syst. t. 129, /. 3, 4, 5.— D. K, Crouch., t. 13,/. 2.— 0. S., 

 Sow. Th. l,p. 366, pi. 73,/. 8. Rather irregularly orbicular, thickish ; 

 apex nearly central ; upper valve with numerous elevated striae from 

 the apex to the margin, f . Mr. Sowerby appeal's to have changed 

 the name from a not unreasonable doubt of the implied locality. 

 Lamarck's pictorial definition of 0. Norwegica points to Crania N. 

 Hah. ? 



D. (0.) Strigata. Brod. Z. T. 1, t. 23,/. 1.— Sow. Th. 1, p. 366, 

 t. 73, /. 7. Rather thick, nearly orbicular, slightly striated, radi- 

 atingly streaked with chestnut-colour ; epidermis thin, pale brown. 

 y 7 ^. Guatemala. l 



THECIDIUM. 2 



Small, thick, punctated, inequivalve, Terebratula-like ; the larger 

 valve attached by the produced umbo, which is imperforate ivhen adult. 

 Smaller valve flattened, opercular ; hinge-area flat, triangular ; interior 

 almost filled by a complicated apophysis. 



T. Mediterraneum. — Thecidea M., Pisso, p. 394,/. 183. — Desh. 

 Lam. 7, p. 346.— Phil. 1, p. 99, t. 6,/. 17.— Thecidium M., Sow. G. 

 —Peeve, t. 127, /. 6, 7.— Sow. Th. 1, p. 371, t. 73,/. 30, 1, 2.— Gray, 

 Brach. p. 118,/. J 26. — Woodw. f. 128. — T. Spondylea, Scacchi. 

 Small, pale yellow, smooth, subquadrate, slightly bilobed ventrally, 



1 See, too, the 0. E vansii of Davidson (Z. P. 1852, t, 14, /. 32, 3, 4), 

 and the O. Antillarum, D'Orb. Mol. Cub. 



z Thecidea of Defrance, admitted as a genus by Deshayes. The 

 internal appendage has been thus described : — " Disk grooved for the 

 reception of the loop, the grooves separated by branches from a 

 central septum ; loop often unsymmetrical, lobed, and united more 

 or less intimately with the sides of the groove." 



