12 ART. 6. M. YOKOYAMA ON SOME 



o. GERVILLIA TRIGONA n. sr. 



PL n, Fig. 7, 8. 



The shell is triangular, inflated and smooth. The l)eaks are 

 prominent, close to the anterior extremity of the straight hinge- 

 line, in which there are four ligamental pits, three of which are 

 just l)elow the l)eak, while the remaining one is situated a little 

 behind it. They are separated from one another by nearly equal 

 distances, although differing in shape the two middle ones l)eing 

 longitudinally elongated, the 2)Osterior transversely elongated, and 

 the anterior obliquely elongated, the upper end being away from 

 the others. The hinge line is anteriorly eared and posteriorly 

 produced forming a large wing pointed at the end. The posterior 

 border is nearly vertical and slightly concave below the hinge- 

 l)order, while the anterior border which is also somewhat ex- 

 cavated in the middle recedes obliquely to a short rounded 

 ventral border, so that the shape of the shell is decidedly 

 triangular. 



The ])roportions of the various diniensions of the shell are 

 best seen in a cast drawn in fig. 7, in which the length of the 

 hinge l)order measures 28 mm., the height of the shell 25 mm. 

 and the thickness of the left valve U) nun. The specimen 

 shown in fig. 8 is more than double the size of the above, but 

 it looks broader than it actually is, as it has l)een distorted by 

 pressure. 



The posterioi- wing of this species reminds one of the genus 

 Pteropeima occurring in the Lower Oolite of France and England. 



Locality : — Hosoura. Tolerably frequent. 



