264 . CALLISTOCHITON. 



Girdle narrow, thin, covered with very densely imbricating 

 minute deeply striated scales (fig. 8.) 



Length 16, breadth 7 mill. 



San Diego, California. 



The form described above was known to Dr. Carpenter by one 

 perfect though worn specimen and a single worn posterior valve. 

 He gives (MSS., p. 134) the following notice of it, under C. palm- 

 iilutus. 



"? Variat. : ar. centr. liris utr. lat. 16-18; v. post. 5, bifurcatis; 

 mucro usque ad dimidium totius altitudinis depresso ; sinus lamina 

 vixfissata. Long. 15, lat. 7*, diverg. 110." 



These lines give the essential points of difference between this 

 variety and the typical form, viz., the more numerous side riblets of 

 the central areas in a shell of the same width, the greatly elevated 

 posterior area of the tail valve, the greater proportionate length and 

 altitude of the shell and the smaller angle of divergence. The value 

 of these characters for distinguishing the two forms remains to be 

 seen, and can only be ascertained by the examination of abundant 

 material. That this form is not merely a fully grown (ephebic) or 

 old (geroutic) state of the original palmulatus is shown by the com- 

 parison of small specimens, not exceeding in length the typicalpafa- 

 11 lat us before me. 



The enormously swollen posterior area of the tail valve separates 

 this from all other species. In several additional details the form 

 differs from C. crassicostatus, notably in the strongly two-ribbed 

 lateral areas, the more numerous ribs of the anterior valve, the 

 riblets of the central areas being parallel at the ridge except on the 

 second valve where they diverge ; whilst in C. crassicostatus they 

 converge forwardly on all of the median valves. The seventh 

 valve of this form is distinctly narrower than the other valves, being 

 crowded by the unusually developed tail-valve. 



The type specimen is in the collection of the Academy. 



C. CRAS3ICOSTATUS Pilsbry, n. sp. PI. 58, figs. 1, 2, 3, 4, 5, 6. 



Shell oblong, elevated, the dorsal ridge very obsoletely angular, 

 side-slopes arched. Surface lusterless, green or brown. 



Valves not beaked, the lateral areas extremely in-ominent. mien ///// 

 granu/'it* /, the concentric riblets being cut by one or several radiat- 

 ing grooves. Central areas having strong longitudinal bars, con- 

 verging \-like on the indge (even on the second valve], the intervals 



