INVERTEBRATE PALAEONTOLOGY. 203 



the beaks in front and posteriorly, the anterior slope being straight, and the 

 posterior a little convex in outline; basal margin forming a regular semi- 

 elliptic curve; extremities subangular ; surface unknown; lateral teeth of 

 hinge long, linear, and compressed. 



Length, 0.93 inch ; height, 0.70 inch. 



Of this species, I have only seen an imperfect cast of the interior of the 

 right valve ; and it is eroded so as to remove the impression of the pallial 

 line, while it gives no idea of the nature of the cardinal teeth. There are 

 impressions, however, of a long, linear, compressed, lateral tooth, parallel to 

 the cardinal margin, both in front of the beak and behind it, like those of the 

 genus Arcopagella* As the cardinal teeth and other characters, however, 

 may be altogether different from those of that genus, I only place it here 

 provisionally until specimens can be found giving more reliable information 

 in regard to its affinities. 



It is proportionally higher and has a different general outline from the 

 last. 



Locality and position. — Big Sioux River, three miles above its mouth; 

 Dakota group of the Cretaceous series. 



MACTRIDiE. 



Genus MACTRA, Linnaeus. 



Synon.— Mactra. Linu. (1767), Syst. Nat., ed., XII, 1125.— MUller (1776), Prodr., xxxi.— Brug. (1789), 

 Encyc. JWtb., I, xiii.— Lamarck (1799), Prodr., 85; and (1801) Syst. An., 121; also 

 (1809), Phil. Zoiil. ; and (1818), Hist., 471.— Feruss. (1821), Tab. Syst,, xliv; also of 

 many other authors. 



Trigonella (sp.), Dacosta (1778), Test. Brit., 196 (not Conrad, 1837; nor Lovdu, 1846). 



Capaterium (part), Mensch., (1787), Mus. Gev., 462. 



Callistoderma (sp.), Poli (1795), Test. Utr. Sic., II, 252. 



Schizodesma, Gray (1837), Loudon Magazine (u. s.), I, 370. — Swainsou (1840), Malacol., 368. — 

 Sowerby, jr. (1842), Couch. Man., 2d ed., 253. 



Scissodcsma, Gray (1842), Synon. Brit. Mus., 75. 



Cymbophora, Gabb (1869), Geol. Report California (Pakeont.), II, 180. 



Etym. — /idnTpa, a kueading-trough. 

 Examp. — Mactra slultorum, Linu. 



Shell ovate-subtrigonal, or trigonal ; hinge with cardinal teeth moder- 

 ately developed, that of the right valve usually smaller than in the left ; lateral 

 teeth elongated, prominent in the middle ; ligament external or marginal, 

 and occupying a triangular groove, opening into, or more frequently separated 

 by a shelly ridge from the cartilage-pit, which is either a mere excavation in 



* The slits or impressions left by these teeth in the cast are not represented near long enough in 

 the engraving. 



i i a a a 



