OF CONCHOLOGY. 247 



ranges through the different members of the chalk formation 

 and contains few species. 



3. Description of a new genus of Mactrid^e. 



MACTRODESMA. Conrad. 



Shell subtriangular ; cartilage pit very large, ovate and pro- 

 jecting much beyond the lower margin of the hinge plate; an- 

 terior hinge margin in the right valve thick and continued much 

 beyond the beak ; hinge of left valve with a profoundly elevated 

 V-shaped cardinal tooth, connected with the hinge line above it 

 only at the base of the tooth ; lateral teeih short, thick, subequal ; 

 pallial sinus narrower and deeper than in Mactra, ending in a 

 line opposite to the middle of the cartilage pit ; muscular scars 

 very large. 



Mactra ponderosa, Conrad. Miocene fossils. — p. 25, pi. 14, 

 fig. 1. 



This large, ponderous shell is very unlike any other form of 

 Mactridce, except in outline. It is abundant on the right bank 

 of St. Mary's River, Md., in Miocene. 



4. Description of a new genus of fossil univalves. 

 HERCORHYNCUS. 



Shell fusiform ; spire prominent, scalariform ; longitudinally 

 ribbed and tuberculated or with tubercles only ; top depressed 

 above the angle or shoulder of the last whorl, which depression 

 becomes angular at the aperture, emarginating the upper part of 

 the labrum ; last whorl broad and rather abruptly rounded at 

 base ; beak rather abruptly recurved and produced. 



Fusus Tippana, Con. (Type). , Journ. Acad. Nat. Sc, second 

 series, vol. iv., p. 286, pi., 4jS, fig.Al. 



Strepsidura Ripleyana, Con. Ibid., fig.i|2. 



This genus is sculptured in a rather peculiar manner, having, • 

 in the only two American species known, either very large 

 tubercles on the angle of the last whorl, or very regular longi- 

 tudinal ribs, with a row of tubercles round the suture. I should 

 not think of placing them in the family Purpuridce, to which 

 Stoliczka refers a species which he names Rapa corallina, and 

 inclines to believe it identical with Fusus Tippana. I believe 

 the two species are not members of one genus. 



