132 CEETACEOUS PELECYPODA 



of IpMgenia, and others, possessing a tliin shell. It is in fact by no means 

 certain that a thorough examination of the animals of the various forms of Donax 

 and Tellina will greatly support a separation into two families, although the typical 

 TellincB and Donaces are apparently more distinct than is ScroUcularia for instance 

 from Gastrana, With respect to form and structure of the shells we see that the 

 Tellinidjs are analogous to the Scrobiculamiibje, while, as already mentioned, 

 the Don ACID ^ exhibit a similar relation to the 'Pa:pb:iidm, On the other hand, the 

 Bon ACID ^ form a good connecting link between the TELLINACEA and the 

 VENEBA OEA, so much so that several authors like Philippi and others consider 

 it more correct to class Venus and allied genera in the family Tjellinidae ; this 

 IS, however, for reasons which I have already (p. 103) stated, by no means justifi- 

 able. The following recent and fossil genera and sub-genera may be conveniently 

 distinguished. 



1. Tancredia, Lycett, 1850, f Hettangia, Ter quern, 1850, see Morris and 

 Lycett, MoUusca of the Great OoL, Pal^ont. Soc, London, 1853, pt. II, p. 90). 

 Shell equivalve, sub-equilateral, anterior side usually somewhat produced, narrowly 

 rounded at the extremity, posterior side shorter, generally sloping and obliquely 

 truncated, ligament short, external ; hinge with one or two cardinal teeth in 

 each valve, a small anterior in the right and a similar posterior in the left valve 

 being sometimes developed; lateral teeth two in each valve, approximate to the 

 cardinals, and usually stronger in the right than in the left valve ; muscular im- 

 pressions oval, pallial line simple. 



This is, as far as I know, the oldest form which certainly belongs to the 

 family Donacibje. The resemblance is indeed so great, that it is almost difficult 

 to distinguish the fossil Tancredice from some Donaces and Ipliigenw. As regards 

 form they resemble both, but as regards dentition they are more allied to Iphigenia, 

 there being either one or two unequal cardinal teeth in each valve. Tancredia is 

 said, however, to possess an entire pallial impression, which is rather remarkable and 

 not to be met with in any of the recent species of Bonacibje^ though it is very 

 small in some belonging to Galatea, 



Species of Tancredia are described from the Lias, Dogger, and Malm ; one 

 species, T. americana, is also noticed from American cretaceous deposits ; but it is 

 very probable that some of the triassic and rhsetic forms have also to be referred to 

 that genus. Meek in his Check list of cretaceous North American fossils proposes 

 for Tancredia a special family which seems to me perfectly unnecessary. Eor the 

 various forms of typical MYA CJEA, some of which have a sinuated and others an 

 entire pallial impression, sufficiently show that we cannot lay too much stress 

 upon that character. 



2. Isodonta, Buvignier, 1852, (Statistique Geol. de la Meuse, p. ii;=? Sower- 

 (/] by a, d'Orb.). Shell equivalve, sub-equilateral, solid, right valve with two almost 



'• horizontally diverging cardinal teeth, left valve with one central cardinal tooth, two 



lateral teeth in each valve much stronger in the right than in the left valve. 

 Isodonta Deshaijesea, Buv., from the Oxford clay, is the type of the genus. 



