AND OTHER FAMILIES. 147 



Unio Haysianus. Plate III. fig. 7. 



Testa subrotundu, subventricosd, ad baseos marginem posteriorem dentatd ; 

 valvulis subcrassis ; natibus prominent ibus ; epidermide luteo-fuscd Iievissimaque ; 

 radiis obsoletis ; dentibus cardinalibus in lobos divisis, lateralibus crassis rectisque ; 

 margarild cacao colore tinctd. 



Shell subrotund, slightly ventricose, dentate at posterior basal margin ; valves scarcely- 

 thick ; beaks elevated ; epidermis yellowish brown and very smooth ; rays obsolete ; 

 cardinal teeth lobed ; lateral teeth thick and straight; nacre chocolate coloured. 



Hab. Cumberland River. Professor Troost. 



My Cabinet. 

 Cabinet of Mr Cooper. 

 Cabinet of Professor Troost, Nashville. 

 Diam. -6, Length -S, Breadth 1 inch. 



Shell subrotund, nearly equilateral, slightly ventricose, dentate at 

 posterior margin, depressed before the umbonial slope ; substance of the 

 shell scarcely thick ; beaks thick and elevated ; epidermis yellowish- 

 brown, very smooth and shining ; rays obsolete ; cardinal teeth lobed, 

 double in the left valve, single and rising from a pit in the right valve ; 

 lateral teeth short, thick and straight ; posterior and anterior cicatrices 

 both distinct ; dorsal cicatrices situated within the cavity of the shell 

 on the plate between the cardinal and lateral teeth ; cavity of the beaks 

 deep and angulated ; nacre chocolate coloured and iridescent posteriorly. 



Remarks. — It has been in my power to examine only four or five 

 specimens of this exceedingly interesting shell. In each of these there 

 is more or less of a dentate appearance, which is so unusual among the 

 Naiades that it may, perhaps with propriety, be said to belong to some 

 American species only. In the early stages of growth there is no den- 

 tate appearance. The U. sulcatus (nobis) and the U. arcaformis 

 (nobis), are frequently furnished with this curious appendage. The 

 dentate variety, mentioned in my description of U. sulcatus, has been, 

 by Mr Say, erected into a separate species, under the name of ridi- 

 bundus; in the propriety of which, however, 1 cannot agree with that 

 naturalist. In outline the present species resembles the U. subrotun- 



