REPTILIAN AGE. JURASSIC PERIOD. 95 



Family TAl^CEEDIID^. 



Shell oval-subtrigonal, longer than high, never very gibbous, equivalve, 

 subeqiiilateral, nearly smooth ; valves gaping or closed ; hinge with car- 

 dinal, and usually posterior lateral teeth ; muscular impressions moderate, 

 smooth ; pallial line simple ; ligament external, or probably sometimes 

 partly internal. 



Animal unknown. 



The genus Tancredia presents a combination of characters that seem to forbid its 

 admission into any of the established families of LameUibrarchiata. Until the affini- 

 ties of this and some little known fossil shells, apparently not more than genericaUy 

 separated from it, can be better determined, it has been thought preferable to pro- 

 pose a distinct family for their reception. It is perhaps most nearly related to the 

 CardiidcB (in which some authors place it) than to any other family, though the 

 more elongate, compressed, Donaciform outline and smooth surface of these shells 

 give them a peculiar physiognomy, very unlike any of the genera known to belong 

 to that family; while some of the closely related fossil forms belonging apparently 

 to one or more undescribed genera are known to be distmctly gaping in front, as 

 well as behind, a feature unknown in the Cardiidce. 



Their simple pallial line, and external ligament, would exclude these shells from 

 the Mactridce, which they resemble in form ; while the former of these characters 

 shows that they cannot be placed in the Tellinidce. Their general physiognomy, 

 not less than their comparatively small, smooth muscular scars, show that they 

 cannot be properly included in the Lucinidce, to which they are often referred. 



This family, in addition to the typical genus, includes the recently proposed genus 

 MeeJaa, of the Cretaceous rocks of California. 



Genus TANCREDIA, Lycett. 



St/non. — Donax, Mactra, &c. (sp.) of Dunker, Deshayes, D'Orbigny, and others. 



Tancredia, Lycett, Ann. Mag. Nat. Hist, vi, 1850, 407. — Morris and Lycett, MolL Great Oolite, 1853, 90. 



Ileitanijm, Terqdem. — Bdvignier, Statist. GeoL MineraL et Palseont. du Dept. de la Mus. 1852, Atlas, 

 14.— Teewem, Bui. Soc. Geol. Fr. torn. 10 (2e ser.), 1853, 364. 

 Etym. — Dedicated to Sir Thomas Tancred. 

 Type. — Tancredia donaciformis, Lycett. 



Shell depressed subtrigonal, or longitudinally subovate, rather compressed, without 

 a defined lunule ; basal margin semi-ovate or semi-elliptical in outline, not crenate 

 within ; posterior side wider and more convex than the other, sometimes gaping ; 

 anterior side more or less attenuate or pointed, and closed; beaks subcentral, 

 usually small, contiguous ; ligament short, external, occupying a small depression. 

 Hinge with one obtuse cardinal tooth in each valve, fitting into a corresponding 

 cavity in the other ; sometimes a small accessory cardinal tooth at the anterior side 

 of the cavity in the right valve, and on the posterior side in the left. Lateral teeth 

 large, obtuse posterior, that of the left valve prominent, and fitting into a depression 

 in the tooth or callosity of the other valve. Muscular impressions oval; the simple 



