262 
PALEONTOLOGY OF NEW YORK. 
rostral region or pedicle-cavity, and produced into divergent ridges, usually two 
on each side, and a fifth in the median axis. These may extend to the mar¬ 
gins or disappear before reaching the middle of the valve and are variously 
subdivided by vascular grooves and sinuses emanating from them. 
In the brachial valve the hinge-plate is small, similar to that of Hens- 
SEL^RiA and Amphigenia in general form, but is of relatively less size than in 
the former genus and is not perforated by a visceral foramen opening beneath 
the apex. Two very narrow, almost linear and closely submarginal dental 
sockets extend nearly to the apex; within them lie two broad, subtriangular 
crural plates, which are divided by a triangular median fissure extending to 
the bottom of the valve. The inner anterior angles of these plates bear the 
slender crural processes, the extent of which is unknown. In mature individ¬ 
uals the apical portion of the hinge-plate is peculiarly constructed; the lateral 
areas become more or less completely united, without altogether obliterating 
the median triangular fissure, and above this point the surface is excavated 
into a spoon-shaped cavity, when the development is extreme, or is trans¬ 
versely angular in the average individual. At a short distance from the 
hinge-plate and in the bottom of the valve there arises a low median ridge, 
which continues for a short distance, separating the obovate, narrowly flabelli- 
form scars of the anterior and posterior adductor muscles. The anterior scars 
are considerably the larger, and their surface is longitudinally striated. The 
vascular grooves and ridges are more obscurely developed than in the pedicle- 
valve. 
Surface smooth or covered with fine concentric striae accompanied by stronger 
wrinkles of growth. The inner laminae are sometimes marked by obscure 
radiating striae near the margins of the valves. 
Shell-substance finely punctate. 
Type, Renssel(zria ? Johannis, Hall. Hamilton group. 
Observations. Several of the earlier species of RensseljERia have an essen¬ 
tially similar form to the representatives of this genus. This, however, is not 
the broad-shouldered form of R. ovoides nor of Amphigenia, and none have 
