GENUS ZYGOSPIRA AND ITS RELATIONS TO ATRYPA. 2G7 



VIII. NOTE UPON THE GENUS ZYGOSPIRA AND ITS RELATIONS 



TO ATRYPA. 



In the Thirteenth Report on the State Cabinet, page 69, I called 

 attention to the existence of internal spires in the fossil known as Atrijya 

 modesta ( Producta modest a, 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 farther 

 attention 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 Zygospira : Shells bivalve, equilateral, inequivalve : surfaces 

 plicate in the typical species ; a sinus in 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, figs. 1 and 2, were given in the same con- 

 nexion. 



Fig.l. Fig. 2. 



Zygospira modesta. Zygospira modesta. 



The internal spires as seen from the The internal spires as seen from the 



ventral side. dorsal side. 



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 

 between the genera still more close. In Zygospira, the apices are more 

 obliquely directed towards the centre of the dorsal valve than in Atrypa; 

 the loop is attached much lower down on the limb of the spire, and is 

 more direct, as is shown in the accompanying figure of Atrypa reticularis^ 



