140 UNITED STATES GEOLOGICAL SUliVEY OF THE TEEEITOKIES. 



TANCREDID.dE. 



Genus TANCREDIA, Lycett. 



Syiwii. — Donax, Maetra, &c. (sp.), Dunker, Deshayes, d'Orbigny, and others. 



Tancredia, Lycett (1350), Ann. Mag. Nat. Hist., VI, 407.— Morris and Lycett (1853), Moll. Great 



Oolite, 90.— Meek and Hayden (1864), Pakeont. Upper Mo., 95.— Stoliczka (1870), 



Palajout. Indica, III, 132. 

 Hettangia, Terqueni (185'2), Statist. Ge'ol. Mineral, et Paloeont. du Depart, de la Mus., Atlas, 14; 



and (1853), Bull. Soc. Ge'ol. Fr., X (2e Be"r.), 364. 



Etym. — Dedicated to Sir Thos. Tancred. 

 Type. — Tancredia donaciformis, Lycett. 



Shell transversely subovate or subtrigonal, moderately convex, or more 

 or less compressed, without a defined lunule ; beaks usually subcentral; pos- 

 terior side often shorter and wider than the other, and more or less truncated, 

 sometimes gaping ; anterior side generally more attenuated, and always closed ; 

 ligament small, short, and external; hinge with one obtuse cardinal tooth in 

 each valve, fitting into a corresponding pit in the other, and sometimes a 

 smaller second cardinal tooth at the anterior side of the pit of the right valve, 

 and on the posterior side of that of the left ; lateral teeth posterior, obtuse, 

 near the cardinal, that of the left valve fitting into a furrow dividing that of 

 the right; muscular impressions oval ; the simple pallial line rather remote 

 from the free margins, which are not crenate within ; surface witli only 

 concentric markings. 



This genus seems to be closely related to Mcehia of Gabb, from the 

 Cretaceous rocks of California, but differs in being closed instead of gaping 

 anteriorly, as well as in wanting the peculiar anterior angularity of that type; 

 which also presents some differences imthe nature of its hinge-plate, and is 

 said to have its ligament subexternal instead of decidedly external. 



The genus Tancredia dates back to the Liassic period ; some ten or 

 twelve species having been identified by Buvignier and Terquem in rocks of 

 that age in France. So far as I am informed, it has not been recognized at 

 any higher horizon than the Lias on the Continent; but in England it is 

 only known to occur in the Lower Oolite. In America it appeals to range 

 still higher, the following species believed to belong to it being from the 

 Cretaceous. In this country, however, it also seems to have made its first 

 appearance during the Jurassic period. Up to this time it is unknown in the 

 Tertiary, or among the Mollusca of the present period. 



