68 Mr. J. Lycett on Fossil Shells from the 



On Tancredia, a fossil genus of Lamellibranchiate Conchifera. 

 Plate II. figs. 8, 9, 10. 



Gen. Char. Shell thin, cqui valve, inequilateral, smooth, flattened, 

 subtrigonal or transverse, somewhat gaping at the posterior 

 extremity, which is produced and attenuated ; anterior side 

 with a longitudinal angle passing from the umbo to the an- 

 tero-ventral border. Hinge with two cardinal teeth in each 

 valve, the anterior one the larger, and a wide and deep, rather 

 irregular fossa between them ; lateral teeth distant, one or two 

 in each valve (usually two); ligament probably partially internal 

 and contained in the cardinal pit. 



The figure of the cardinal pit varies in the different species ; 

 in one it is triangular, one of the angles being at the umbo, in 

 others it is wider and more irregular, but there is not any raised 

 edge bordering it, as in Mesodesma and the Lutrarice ; the figure 

 and size of the cardinal teeth likewise vary ; occasionally the pos- 

 terior cardinal tooth can hardly be distinguished ; strictly speak- 

 ing, the anterior cardinal tooth is immediately beneath the umbo, 

 the pit and other tooth being posterior to it ; the posterior late- 

 ral tooth is sometimes wanting altogether ; the internal margins 

 of the valves are smooth ; the valves are thin and delicate, but 

 such as have had their internal surface exposed showed no traces 

 of the muscular impressions. This genus may be classed as one 

 of the Mactracea, and placed near to Mesodesma and Amplddesma ; 

 the external figure is donaciform ; the character of the dentition 

 approaches near to, but is really distinct from, Mesodesma, from 

 which latter genus the gaping posterior extremity tends to sepa- 

 rate it ; the shell is likewise thinner and more delicate than in 

 either of the genera with which it has been compared ; with Donax 

 it has nothing more in common than the external form. 



This genus of small bivalves is eminently characteristic of the 

 lower members of the oolitic system of rocks ; the Great Oolite 

 has three species, and the freestone beds of the Inferior Oolite 

 have two other species ; neither of these are common to the two 

 formations, nor have they been found in the upper or lower di- 

 visions of the Inferior Oolite. The diffusion of this generic form 

 i> worthy of notice ; it may without exaggeration be said, that cer- 

 tain layers in the shelly portion of the Great Oolite were merely 

 so many colonics in which they propagated almost exclusively in 

 countless numbers, but the great mass of these are of one species ; 

 the freestone beds of the Inferior Oolite contain likewise a great 

 number of another species. A knowledge of these five species is 

 of importance in the recognition and distinction of the shelly 

 beds in the two formations, as from the numbers of two or three 



