INVERTEBRATE PALEONTOLOGY. 117 



also marked with costae, radiately arranged on the Hanks and behind, and 

 divaricating in front. 



It is doubtful whether there are any true Crassatellce older than the 

 Jurassic period, and even the species of that age referred to this genus may 

 belong to other perhaps undefined genera. We see the name sometimes 

 used even for Palaeozoic shells, but it is now only done by those who are 

 rather careless in regard to generic distinctions. 



In the Cretaceous, however, there are a number of well-marked species 

 of Crassatella, belonging, however, to Mr. Conrad's section Pachythcerus ; 

 the same group is also represented in the Eocene, while those of the Miocene 

 and existing seas seem nearly or quite all to belong to the slightly different 

 typical section. The number of species was perhaps greater during the 

 Eocene or Cretaceous than at present. The recent species are generally 

 obtained from deep waters on the coasts of Australia, Asia, Africa, North 

 America, &c. 



Crassatella (Pachythxrns) Evan si, H. & M. 



Plate 17, figs. G, a, h, c, d. 

 Crassatella Evan si, Hall and Meek (1854), Mem. Am. Acad. Arts and Sci. Boston, V (n. s.), 383, pi. 1, fig. 9. 



Shell ovate-subtrigonal, rather gibbous, thick; anterior side obliquely 

 truncated, and generally a little concave in outline above, and abruptly rounded 

 below ; base semi-ovate, being usually slightly more prominent before than 

 behind the middle; posterior side longer than the other, sloping from the 

 beaks above, and rounded or vertically subtruncate at the extremity; lunule 

 oval, rather deep ; escutcheon narrow-lanceolate, defined by an obscure ridge 

 on each side ; pallial margins regularly and finely crenate within ; beaks 

 prominent, gibbous, and located about half-way between" the middle and the 

 anterior extremity. Surface marked by fine, irregular lines of growth, which 

 are sometimes gathered into small, obscure wrinkles near the borders; 

 muscular and pallial impressions strongly marked. 



Length, 1 inch; height, 90 inch; convexity, 0.71 inch. 



Decomposing and exfoliated specimens of this species show on the 

 surfaces of the inner laminae distinct radiating lines. These were evidently 

 left by the crenulations of the border, which, as may be seen by their marks 

 on the edge of each lamina of growth, marked the pallial margin of the shell 

 at all ages. 



