50 DESCRIPTION OF NEW 



VII., is undoubtedly such. It may be observed, in each of these figures, 

 that there is an enlargement at the posterior basal margin. This, so 

 far as my knowledge extends, is always indicative of the female cha- 

 racter in this form of shells, and this enlargement of the valve cor- 

 responds with the position of the charged oviducts. In the cariosus 

 1 have observed frequently, that the oviducts WTre so full as to be pro- 

 truded w^hen the animal was at rest, and when disturbed it retracted 

 them with sluggishness. In a single case of a female radiatus which I 

 touched in the river, the oviducts were visibly outside of the line of 

 the shell, and the sudden alarm caused so rapid a closing of the valves 

 as to cut off several of the sacks, which floated away. 



There are a few species very remarkable in the shell for a charac- 

 ter, which I refer to sex entirely, but never having had the advantage 

 of examining the anatomy of the included animals, I do not present it 

 as an undoubted fact. In my description of U. arcseformis, Trans. Am. 

 Phil. Soc, Vol. IV., page 116, I noticed the enlargement and dentate 

 appearance of the posterior margin. In this species, the brevidens. sul- 

 catus,* capillaris, and triangularis,^ we find an enlargement to com- 

 mence at a middle age before or along the umbonial slope, and each 

 mark of growth on this enlargement is disposed to be dentate. In the 

 brevidens it is so abrupt as to. resemble a large cord on the inferior por- 

 tion of the valves. In the arcseformis it is somewhat flattened and but 

 slightly influences the plane of the margin. In the capillaris it is more 

 spread out, but the enlargement and dentition are still very percepti- 

 ble. One of my specimens of the sulcatus presents the dentitions 

 more complete than I have observed in any other species. In all the 

 species where this enlargement takes place, a corresponding groove may 

 be observed in the interior part of the valve. In corroboration of my 

 conclusion, that these are female shells. \ve have specimens of full 

 growth of undoubtedly the same species which have not the least ap- 

 pearance of an enlargement; and all those which have not attained more 

 than one-third or half their growth never in any case present it. 



• See my remarks, Vol. III., p. 431. 



t The male differing so much in form in the posterior part, induced me to think it to be a 

 distinct species, and as such, I described it ia Vol. IV., p. Ill, under the name oiformosvs. 

 See Plate XVI., fig. 41. . 



