MUSEUM OF COMPARATIVE ZOOLOGY. 271 



usual brown or yellow markings ; have the umbones of a deep pink, fading oflF 

 on the dome of the valves, and a little more elevated than in specimens from 

 shallow water; they do not exceed 15.0 mm. in height, and may be considered 

 as forming a variety syhariticum. 



Family ISOCARDIID^. 



Gends ISOCARDIA Lamakck. 



Subgenus MEIOCARDIA H. & A. Adams. 



Meiocardia Agassizii, n. s. 



Shell subquadrate, widest at the gently arched base, polished, moderately 

 inflated, whitish or yellowish; beaks small, incoiled away from the hinge 

 margin, with a depressed but not circumscribed area in front of them; from 

 the beak proceeds a strong but moderately sharp-edged keel to the lower 

 posterior end of the shell, which is produced in a rather sharp angle ; on the 

 posterior area thus separated near its inner edge is another keel, less sharp, 

 which extends from the beak to the somewhat rounded upper posterior angle 

 of the shell, from which the margin obliquely descends with a slight con- 

 cavity first, and then a little convex, to the lower angle; between the two 

 keels and nearer the anterior one is a single rounded rib extending to the 

 margin, over which the concentric sculpture is continuous and unmodified; 

 the entire surface is sculptured with fine even concentric rugae, somewhat obso- 

 lete toward the umbo and a little stronger behind the principal keel; toward 

 the margin are occasional impressed lines indicative of changes in the rate of 

 growth and presumably varying with the individual; the anterior end of the 

 shell descends toward the base, where it is somewhat pointedly rounded, and 

 from which the base extends in an easy outward curve to the posterior angu- 

 lation; pallial line faint, simple, margin of the shell plain; hinge much as in 

 M. vulgaris Keeve, the cardinal tooth in the right valve rounded behind, gently 

 indented on each side ; ligament external, in a groove with raised edges, con- 

 tinued under the beak as in other species. Lon. of shell 22.0; max. alt. 17.0; 

 diara. of right valve 8.0; of whole shell, probably 16.0 mm. 



One right valve dredged off Trinidad by the U. S. Fish Commission in 117 

 fms., bottom temperature 64^.5 F. 



The great interest attaching to this species, not only on account of its ap- 

 pearance in a new faunal region for the genus, but on account of the very 

 small number of recent species known, has led me to insert it here, though not 

 obtained by the " Blake." * It is dedicated to the memory of my teacher, the 

 late Prof. Louis Agassiz, whose work in the molluscan subkingdom is familiar 

 to alL " Cypricardia" isocardioides of Deshayes (Inv. Bas. Paris) would, from 

 the figure, fall into line with M. Agdssizii. It is certainly not a Cypricardia. 



