OF CONCHOLOGY. 11 



GONIOMYA, Agass. 



G. APERTA, Gabb. — PI. 6, fig. 8. 



Shell very thin, subcompressecl, gaping posteriorly ; beaks 

 small, prominent and placed about two fifths of the length from 

 the anterior end ; cardinal margin rapidly sloping, with a slight 

 convexity in advance, posteriorly concave at first, and running 

 out nearly straight to the posterior end ; anterior end narrowly 

 rounded and prominent ; posterior subangulated, being obliquely 

 truncated above and below and produced in the middle; base 

 broadly convex. Surface ornamented by numerous concentric 

 ribs, broadly angulated, the angle being a little posterior to the 

 beaks. These ribs are very uniform in size and, commencing at 

 the anterior end, are coincident with the lines of the growth for 

 about a third of their length, they then bend towards the base 

 very slightly, become somewhat undulated and, when nearly 

 one-third of the length from the posterior end, bend up somewhat 

 abruptly, gradually resuming the direction of the stride of growth, 

 and finally disappear near the cardinal margin. 



Length 2-4 in., width 1*4 in., diameter of the valves -9 in. 



Locality. — With the preceding. Mr. Clayton. 



This shell is more evenly ribbed, and the angle of the ribs is 

 wider than in any other species with which I am acquainted. 



MYACITES, Munst. 



M. DEPRESSus, Meek. 



Mijacitcs depressus, Meek, Pal. Cal., vol. I, p. 51, pi. 8, fig. 



6, 6 a. 

 A single specimen of this shell was found by Mr. Clayton at 

 Volcano. 



CARDIUM, Linn. 



C. ARC^FORMis, Gabb. — PI. 3, fig. 9. 



Shell of moderate size, very thick, convex, oblique, longer 

 than wide ; beaks prominent, strongly incurved, in some cases 

 almost in contact, placed about a third of the length from the 

 anterior end, which is regularly rounded ; posterior end variable, 

 rounded or obliquely truncated ; base pretty regularly and broadly 

 convex. Surface ornamented by about l8 to 20 large convex, 

 radiating ribs with interspaces of about equal size, all crossed by 

 fine but distinct lines of growth. From 11 to 12 of these ribs 

 occupy the anterior and lateral portions of the surface; the re- 

 mainder, on the posterior face, are always smaller and, in some 



