CONTRIBUTIONS TO PALAEONTOLOGY. 



307 



VII. NOTE UPON THE GENUS ZYGOSPIEA AND ITS RELATIONS TO ATRYPA. 



In the Thirteenth Report on the State Cabinet, page 69,1 called attention to 

 the existence of internal spires in the fossil known as Atrypa modesta 

 (Producta modesta, Say sp.), which appear to be arranged as in Atrypa. 

 Not being entirely satisfied with the observation I was then able to make 

 upon the few specimens available, I proposed to give some further atten- 

 tion to the subject in the next Report. At the same time I added a note, 

 asking for information or specimens that might aid in the elucidation of 

 this and other forms. Dr. Rominger, of Ann Arbor, Michigan, responded 

 very generously to my request ; but it was not until the Fifteenth Report 

 (1862) that I was able to give the illustrations of the internal structure 

 in a satisfactory manner. In this Report I remarked that " the direction 

 of the spires is nearly the same as in Atrypa, differing in the presence 

 of the strong loop ; while the shell, in its exterior character, is quite 

 unlike Atrypa." 



I gave the following generic description : 



" Genus Zygospiea. — Shells bivalve, equilateral, inoquivalve ; surfaces plicate 

 in the typical species ; a sinus iu the dorsal valve. Internal spires arranged 

 somewhat as in Atrypa, with a broad loop passing from the outer limbs 

 of the spiral band entirely across from side to side, near to or above the 

 centre, and close to the. inner side of the dorsal valve." 



The following illustrations of Zt/gospira modesta were given in the same 

 connexion. Fig. 1 represents the internal spires as seen from the ventral 

 side ; fig. 2, the spires as seen from the dorsal side. 



Fig. 1. 



Fig. 



At that time the spires of Atrypa were not known to be connected 

 by a loop, as has since been ascertained ; and this renders the analogy 



