AND OTHER FAMILIES. 183 



are almost flat. The deposit of the nacre after this forms an angle of 

 nearly 45=' with the surface which it has left, thus forming a hump, or 

 obtuse angle point, directly on the umbo. This causes the curious 

 result, that when the shell is from one third to three fourths grown, 

 it will rest, when so placed, on the portion of surface between the 

 point of the beak and the umbo, the basal margin remaining in the air. 

 In its general characters it resembles the irroratus, but may at once be 

 distinguished by the hump. It is devoid of tubercles, while the irro- 

 ratus is sometimes covered with them, particularly on the posterior 

 part. It differs somewhat also in the rays, the spots in those of the 

 dromas being larger, and generally better defined. The outline differs 

 in being less elongated, being disposed to be more oblique or more 

 transverse. In regard to the structure of the animal, I am not prepared 

 to say that it differs from that of the irrorahis.* Not having had an 

 opportunity to examine the animal, I can only judge by analogy, which 

 would, I think, induce one to conclude that the same curious pendent 

 oviducts would be found in both. I hope to be able to procure from 

 professor Troost a specimen in that period of gestation. 



Unio Troostensis. Plate X. fig. 30. 



Tesid scalend, cuneatd, obliqud, valde inxquilaterali ; valvulis antice crassiori- 

 bus; natibus subterminalibus ; epidermide luieold, 7-adiis capillaribus multis ; den- 

 tibus cardinalibus elevatis, cristatis ; lattralibus subrectis ; margaritd albd et iri- 

 descente. 



Shell scaleniform, wedge shaped, oblique, very inequilateral ; valves thicker ante- 

 riorly ; beaks nearly terminal ; epidermis yellowish, filled with numerous capillary 

 rays ; cardinal teeth elevated, crested ; lateral teeth nearly straight ; nacre pearly white 

 and iridescent. 



Hab. Cumberland River. Professor Troost. 



My Cabinet. 

 Cabinet of Professor Troost. 



See ante page 13. 



