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 incurved, 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. harrisi , 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 lamellae, in the course 
of the first volution, are united by a loop 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 Athyris, Spirigera, Merista, etc.; strenuously objecting 
to the introduction of the generic name Meristella. At the close of the same article, he proposed 
[ Paleontology IV.] 38 
