GENUS MERISTELLA. 297 



The interior of the dorsal valve is marked by the presence of a strong 

 hinge-plate or cardinal process ; and from the base of this proceeds a 

 thin longitudinal septum, which often extends for half the length of the 

 valve. 



The interior of the ventral valve shows a triangular fissure below the 

 beak, which joins a semicircular perforation at the apex. At the base of 

 this fissure are two strong teeth, which extend in thickened or slender 

 plates to the bottom of the cavity, and curve around the upper part of. 

 the muscular area, which is broadly triangular or ovate. 



There is sometimes a thickening of the shell at the base of the rostral 

 cavity, which abruptly limits the muscular impression; but there is 

 neither septum nor rudiment of one as in Merista. 



In well;preserved specimens of M. haskinsi, where the apex is not too 

 closely in,curved, the ventral beak has a circular foramen, and the triangu- 

 lar space below, which is usually filled by the beak of the dorsal valve, 

 is closed by two deltidial pieces anchylosed in the centre. The latter 

 feature has been observed in M. barrisi, and probably existed in all the 

 species at some period of their growth. 



The study of the interior has shown that the thickened bases of the 

 crura extend forward for a short distance, or bend abruptly to the 

 ventral side, but recurving, descend into the cavity of the dorsal valve, 

 following its contour and that of the ventral valve in their succeeding 

 volutions. In the bottom of the dorsal valve, the lamella?, in the course 

 of the first volution, are united by a loojj which is produced by the 

 extension of a slender process from the band on each side, and these are 

 united at a greater or less distance from their origin. Beyond this junc- 

 tion the parts of the loop again divide, and each one is produced in a 

 curving band which arches forward on the ventral side, and thence 

 returning is reunited to the sides of the loop at or near the junction of 

 the parts before noticed.* 



• In the Canadian Journal of Industry Science and Art, for March, 1861, Mr. Billings has under- 

 taken to define the limits of the Genera Athtris, Spibigbea, Mekista, etc. ; strenuously objecting 

 to the introduction of the generic name Meiiistella. At the close of the same article, he proposed 



[ Pal^ioxtology IV.] 38 



