NATURAL SCIENCES OF PHILADELPHIA. 83 



which is on the posteriox* side of the primary impression. The 

 latter is very much smaller than in an Anodonta of the same size. 



H. capax. PL 12, figs. 1, 2, 3. * 



Ovate,? thick and ponderous, inflated; umbo prominent, 

 rounded ; beaks distant from anterior extremity ; valves some- 

 what contracted near the middle towards the base. 



Prof. Orton remarks that he saw many fragments of this shell, 

 and, therefore, it probably lived in company with Pachydon, but 

 the lamina of the shell separate so readily that fragments only are 

 obtainable. The shape of the shell is evidently somewhat like 



that of a TRIQUETRA. 



HEMISINUS, SwaiDson. 

 H. tuberculiferus. PI. 12, fig. 4. 



Turreted, elongated, volutions 9, laterally straight ; 3 prominent revolv- 

 ing tnberculated ribs on each volution of the spire, except two or three 

 nearest the apex ; last volution with 9 or 10 revolving lines, unequal in 

 size ; a fine carinated line borders the upper margin of the suture, 

 which is indistinctly defined ; aperture short. 



A beautiful species, very distinct from any living shell of the 

 genus. It occurs often very perfect, with the exception of the 

 labrum, which is broken, in every specimen I have seen, from the 

 chalky nature of the shell, not generally from attrition. 



PACHYDON, Gabb. 

 P. tenuis. PI. 12, fig. 5. 



Outlines representing extremes of variation. 

 P. . PI. 12, fig. 6. 



As there is only one specimen of this form, I am uncertain 

 whether it is a distinct species or a variety of P. tenuis. 



In my last paper, for Mylitoides read Mytilopsia. 



