74 ISCHNOCHITOX. 



Exped. p. 321, t. 433 (bad). Trachydermon dentiens DALL, Proc. 

 U. S. Nat. Mas. 1878, p. 323. Tracfiydermon pse'udodentiens CAR- 

 PENTER, Brit. Asso. Adv. Sci. Rep. 1863, p. 649 ; Proc. Acad. Nat. 

 6ci. Phila. 1865, p. 60. 



This species is closely allied to cinereus L., but differs in the 

 sharper granulation, the granules being less regularly arranged, in 

 the form of the sutural plates and the color both externally and 

 within. The girdle of dentiens differs from that of cinereus in 

 lacking the marginal spicules characteristic of that species. The 

 gills are long, as in cinereus. 



Gould's diagnosis and figures are extremely incorrect, the light 

 spots along the sutures having been mistaken by him for projections, 

 or "teeth," whence the name dentiens. Carpenter ascertained the 

 identity of dentiens with the present form by an examination of 

 Gould's type specimen. 



A series from San Ignacio Lagoon, Lower California, differs from 

 the San Diego shells in being narrower, chiefly dark green (occa- 

 sionally with some lighter valves), and in being more subtly granu- 

 lated. The interior is vivid dark green. One of these is shown in 

 fig. 65. These specimens show a short, slightly acute and beaked 

 umbo on the central valves. In the larger northern specimens this 

 is lost by erosion. 



I. GOTHICUS Carpenter. Unfigured. 



Shell small, much elevated, green, elegantly tinted with rose and 

 olive ; valves gothic-arched, the dorsal ridge acute ; lateral areas 

 small, arcuately distinctly defined, granulose; umbones prominent; 

 umbonal margin having a tessellated color pattern and incurved. 

 Central areas longitudinally ribbed, ribs rounded, close, not much 

 elevated, the interstices small, sometimes slightly decussated. End 

 valves sculptured like the lateral areas, the tail valve having the 

 umbo median and somewhat elevate:!. 



Interior having the sutural plates scarcely separated, the jugal 

 sinus very broad and shallow ; insertion plates of median valves 

 having a single slit, terminal plates with 8-10 slits ; the plates are 

 obtuse; eaves hardly elevated. 



Girdle most minutely scaly, the granules very close, rounded 

 smooth ; edge with small suberect hairs. (Cpr.) 



Length 5, breadth 2* mill. ; div. 80. 



Catalina Island, California. 



