GENUS ATRYPA. 313 



SuEFACE smootli, striate or costate, and often strongly imbricated by squa- 

 mose lines of growth, whicli 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 is 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 forrh 

 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. Whitf'ield 

 that the short processes usually represented near the base of the crura in the spires of Atryi>a are 

 directed into the cavity of th? 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 Mrypa reticularis 

 (See Nineteenth Keport on State Cabinet). I have also received, some time smce, from Dr. C. Romijt- 

 OBB, of Ann Arbor, Michigan, a specimen of the /flrypa nodostriata, showing the spires, and which by 

 farther cutting has revealed the connecting loop. A specimen of ^. 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 ^halcedonic quartz ; and another specimen from the same locality has, by a careful reinovai 

 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 Jlr. 0. St. 

 John. 



[pal.sostologt iv.] 40 



