T8 
PALAEONTOLOGY OP NEW-YORK. 
&EAUS STROPHODOKTA ( Hall). 
The Genus Strophodonta was originally proposed for strophomenoid 
shells, having a crenulated hinge-line, without foramen in the area of 
the ventral valve. The internal characters were not at that time fully 
known, and therefore could not be included in the description. 
In S. demissa, which was regarded as the type of the genus, the area 
of the ventral valve is free from any indication of foramen ; the dental 
lamellae are partially or essentially obsolete, and the divaricator muscular 
impressions are spreading flabelliform without limitation by an elevated 
ridge. In the dorsal valve, the cardinal process is bifurcate from its origin, 
and is directed backwards beneath the area of the ventral valve. On the 
lower side of this ventral area there is a bilobed callosity or process 
which is more or less prominent and embraced by the divisions of the 
cardinal process of the opposite valve; thus strengthening the articulation 
of the valves, or assisting in keeping them in position. The divisions 
of the cardinal process often extend so far beneath the area as to form 
conspicuous pits in the substance of the shell within the umbonal cavity. 
The valves are so closely articulated by the interlocking of these pro¬ 
cesses, that the separated ventral valves are usually broken in the mid¬ 
dle of the area : this happens also to other species, and it is compara¬ 
tively rare to find the inner margin of the ventral valve entire. This 
accident is probably caused by the pressing or crushing of the dorsal 
valve into the cavity of the ventral valve, while the central portion of 
the two, remaining firmly united, carries away the middle of the ventral 
area, which is thinner on each side of the median. 
In the S. reversa of Iowa, this bilobed process of the ventral area is 
very conspicuous; and „ in some specimens of this and of S. lepida, the 
central process remains very prominent, while the margin of the area 
on either side is indented by the branches of the cardinal process of the 
opposite valve. In these species we not only have no evidence of trian¬ 
gular foramen, but there is an absolute thickening on the lower side and 
