NATURAL SCIENCES OF PHILADELPHIA. 29 



DRESSEINA, Vanbeneden. 



Subgenus Mytiloides, Conrad. Pkaxis, H. & A. Adams. 

 D. scripta, Conrad, PI. I., figs. 12, 16. 



Triangular, elevated, ventricose anteriorly, compressed towards 

 the posterior end ; umbo sharply angulated anteriorly, anterior 

 side flattened or concave; markings zigzag brown lines. 



This species much resembles D. polymorpha in its colored mark- 

 ings. It is a much smaller species than the latter, and propor- 

 tionally shorter or more elevated. It is sufficiently abundant to 

 mark the estuary character of the bed in which the species lived. 



Figs. 12 are young shells showing extremes of variation in form. 



There is a small species of this subgenus living in the waters of 

 Virginia, among the oysters planted in brackish waters ; D. leuco- 

 phxata, Conrad. (D. Americana, Recluz.) 



ANODOXTA, Cuvier. 

 A. pebasana, Conrad, PI. I., fig. 5. 



Rhomboidal, elongated, compressed ; dorsal and ventral mar- 

 gins parallel; summit very small and acute, little prominent above 

 the hinge line, and erect ; distant from anterior end. 



A single valve of this species is before me. It is only remark- 

 able for its comparatively low elevation or height. There is a 

 fragment of a much larger species in the collection, which may be 

 A. Batesii, Woodward. The fragment I noticed as allied to Mul- 

 LERiA, Woodward states may be an Anodon, but the extraordinary 

 muscular impression renders that impossible. 



TRIQUETRA, Klein. HYRIA, Lam. 

 T.longula, Conrad, PI. I., fig. 10. 



Subrhoraboidal, slightly ventricose, elongated, scarcely alated, 

 hinge and ventral margins nearly parallel ; anterior margin 

 slightly eraarginate; dorsal line convex, ventral margin nearly 

 straight ; umbonal slope rounded except on the umbo, where it 

 is subangular ; umbo flattened, with longitudinal and transverse 

 zigzag plications ; posterior margin obliquely truncated ; cardinal 

 teeth of right valve, four, the anterior one elongated and com- 

 pressed, rectilinear, two diverging teeth immediately under the 

 apex, and a much less elevated linear oblique tooth between two 

 anterior teeth ; in the left valve two very oblique diverging teeth 



