132 



GENUS CRYPTONELLA, Hall, 1861. 



Shells terebratnliform, equilateral, inequivalve, elongate 

 or transverse, ovoid or sublenticular in form, without 

 median fold or sinus, or with these features very slightly 

 developed towards the base of the shell. Ventral valve 

 with the beak extended or incurved, and terminated by 

 a circular foramen which is limited on the lower side 

 by two small triangular deltidial pieces (these are some- 

 times not visible externally, and the lower side of the 

 foramen is concealed by the umbo of the opposite valve). 

 Shell-structure finely punctate ; surface marked by fine 

 concentric striae, which are sometimes obsolete. Valves 

 articulating by teeth and sockets, the dental lamellae of 

 the ventral valve extending in thin vertical plates into 

 the cavity of the valve. The muscular impressions of 

 the dorsal valve are strongly marked above, and extend 

 in two narrow, gradually widening impressions, more 

 than halfway to the base. The ventral valve shows 

 elongated muscular and vascular impressions below the 

 rostral cavity. 



In the dorsal valve, the hinge-plates, or bases of the 

 crura, support a slender loop, the two limbs of which are 

 flattened, with the faces vertical ; and in its extension for- 

 ward, the upper margins are inclined towards each other, 

 and, gradually widening, become joined, and thence extend- 

 ing forward, form a single lanceolate plate, which may be 

 more or less attenuate in front. These laminae of the loop, 

 after becoming thus conjoined and spreading laterally, are 

 abruptly deflected in a vertical plate along the median line, 

 extending into the cavity of the ventral valve, as shown 

 in figure 2, while looking upon the dorsal side of the loop 

 this vertical plate will be seen projecting backwards be- 

 tween the bands of the loop as well as extending in front, 

 as shown in fig. 12. 



