399 PALAEONTOLOGY OF NEW-YORK. 



Gems Cryi»toxella (Hall, 18G1). 



Crtptokilla : Hall, Fourteenth Report on tlie State Cabinet, p. 102. 1861. 



" Id. Fifteenth " •• •• pp. 132 & 133. 1862, 



" " Sixteenth •• " " p. 48. 1803. 



Shells equilateral, inequivalve, elongate, oval or ovoid ; valves unequally 

 convex, without median fold or sinus,jor with this character moderately 

 developed and principally towards the base of the shell. Ventral valve 

 with beak extended or incurved, perforate; foramen terminal, the 

 lower side formed by two small triangular deltidial pieces, or, in their 

 absence, by the umbo of the opposite valve. Shell-structure finely 

 punctate. Surface smooth, or with concentric strios. Valves articula- 

 ting by teeth and sockets, the dental lamellae of the ventral valve 

 extending downwards into the cavity of the shell. The muscular 

 impressions in the dorsal valve are strongly marked above, and extend 

 in two narrow separated impressions more than halfway to the front 

 of the shell : the ventral cast shows elongated muscular and vascular 

 impressions. 



The species of this genus are more elongate than Merista and Meri- 

 STELLA, and those now known are less distinctly marked by mesial fold 

 and sinus; while the beak is more attenuate, often a little flattened, and 

 rarely so closely incurved, as in the genera cited. The punctate structure 

 of the shell is a distinguishing character. 



This genus was first described as above cited, and figures of the exte- 

 rior form and of the interior of the valves were given in the Fifteenth 

 Report. The results of some farther investigations were given in the 

 Sixteenth Report, and an illustration of Avhat was supposed to be the 

 internal appendcages of the shell. About the same time it was discovered 

 that some punctate shells of the same general form possessed the loop of 

 Terebratula proper ; and it became a matter of great interest, and still 

 greater difficulty, to determine the internal structure of the species. I 

 had referred to this genus the Terebratula lens, T. lincklaini, T. rectirostra 



