GENUS ATRYPA. 
313 
Surface smooth, striate or costate, and often strongly imbricated by squa- 
mose lines of growth, which are sometimes produced in foliacious expan¬ 
sions, nodes, or tubular spines. Structure fibrous. 
In the ventral valve there is a strong tooth on each side at the base of 
the broad fissure: these teeth are somewhat bilobed at their summit, with 
a broad crenulated groove on the back: from the base of the teeth a curving 
ridge extends forwards, and partially encloses a broad muscular area. 
In the dorsal valve, the hinge-plate i^ usually or always divided in the 
middle, with a distinct toothlike plate on each side, and the crura origina¬ 
ting on the outside of these, close to the dental sockets; while on the outside 
of the latter, close to the shell margins, there is on each side a crenulated 
fold, which occupied the groove at the base of the tooth, and this appears 
to be of generic significance. The spires originating from the crura form 
two large hollow cones which are directed into the cavity of the dorsal 
valve, their adjacent sides being flattened, and the apices brought close 
together near the centre of the bottom of the cavity. The extreme gib¬ 
bosity of this valve in many of the older shells gives great space for the 
development of the spires. 
The crura are usually represented as having a pointed process from near 
the base of the spires on each side, directed towards the centre. These 
processes, however, unite in the cavity of the dorsal valve to form a loop 
connecting the spires.* 
* While this volume has been passing through the press, it has been shown by Mr. R. P. Whitfield 
that the short processes usually represented near the base of the crura in the spires of Atrypa are 
directed into the cavity of the dorsal valve, and are there united to form a loop, and that the character 
of this loop varies in different species, or in forms recognized as simple varieties of Atrypa reticularis 
(See Nineteenth Report on State Cabinet). I have also received, some time since, from Dr. C. Romin- 
ger, of Ann Arbor, Michigan, a specimen of the atrypa nodostriata , showing the spires, and which by 
farther cutting has revealed the connecting loop. A specimen of A. reticularis, sent me by Dr. Knapp, 
of Louisville, Ky., from the Falls of the Ohio, preserves the spires and connecting loop, all heavily 
covered by chalcedonic quartz; and another specimen from the same locality has, by a careful removal 
of the ventral valve, revealed the spires and loop. The same features are also shown, although in a less 
perfect manner, in a silicified specimen from the Hamilton group of Iowa, received from Mr. 0. St. 
John. 
[Palaeontology IV.] 
40 
