OF THE GENUS USIO. 263 



&e. by the centre, and in this certainly serves a very useful 

 purpose. 



Being exceedingly anxious to examine the animals of the 

 various species of the Uuio from the Ohio, my brother. 

 T. G. Lea. kindly sent me thirteen species and many varieties, 

 which, %^'ith the assistance of jMr Stewart were carefully dis- 

 sected. Those consisted of the species mytiloides and meta- 

 nevra of Rafinesque : siliquoideus, triangularis, gibbosus and 

 comutus of Bai'ues : purpureus, alatus. ovatus of Say : ^sopus 

 of Green ; irroratus and ellipsis iiow first described. This ex- 

 amination furnished me ^Aith several interesting results. It con- 

 firmed me entirely in my belief that the oviducts of the irro- 

 ratus were different from any other species yet examined ; a 

 drawing and description of which \^^ll be found in this paper. 

 The prolongation of the sacks of the oviducts is peculiarly 

 interesting. In some of the varieties of the cornutus, which 

 seem to run into the ^sopus, we found the posterior and infe- 

 rior parts of the shell unnaturally extended. The mark of 

 the animal on the shell had its usually curved shape, while 

 the mantle, quite callous, extended to a protruded and irregu- 

 lar margin. 



It has been a matter of speculation how the calcareous 

 matter was secreted to increase the outer margin of the teeth 

 as well as their whole surface. In this examination we foimd 

 the surface of the broad teeth, some of which were near half 

 an inch thick, to be completely covered ^^-ith a prolongation 

 of the mantle, extending from the great anterior to the great 

 posterior cicatrix ; so that when the teeth closed they completely 

 enveloped it. This part of the mantle is exceedingly thin and 

 transparent. 



In the study of this genus, we ai'e naturally attracted 

 by the beautiful rays which frequently are found in the 

 epidermis. This to the unpractised eye would seem to 

 be a sufficiently distinctive characteristic to mark a species. 

 There is, however, no character more fleeting and various. 

 The young of many species uniformly possess rays, and we 

 sometimes find fine adult specimens of extreme beauty. The 



