BIVALVIA. 145 



both valves is furnished with two teeth, one simple, the other bifid, the simple one is 

 placed before the umbo in the right valve, and the bifid one is anterior in the left, 

 while the ligament occupies a position wholly external, and is deeply inserted : 

 the muscle marks are large and well impressed, of an oblong form, with the mantle 

 mark entire : numerous fine radiating strife are often visible in the interior, like some 

 of the Lucinee, the outside is what may be called smooth, having only the irregular 

 lines of increase. The shell is somewhat flattened, though occasionally tumid, more 

 especially on the posterior side. 



I have followed Philippi in assigning the Mediterranean shell to this species, as 

 he has done in his second volume, the figure in the first volume moi'e resembles the 

 next species, for which it was taken when my Catalogue was compiled. 



2. DiPLODONTA DILATATA, S. Woocl. Tab. XII, fig. a, b. 



DiPLODONTA DILATATA. S. TFood. Catalogue, 1840. 



— — ? Nyst. Coq. Foss. de Belg., p. 138, pi. 7, fig. 1, 1844. 



— — J. Sow., in Dixon. Geol. and Foss. of the Tert. and Cret. Form. 



of Sussex, p. 167, t. 3, fig. IG, 1850. 

 Venus fragilis? Nyst and JVest. Nouv. Rech. Coq. Foss. d'.4nv., p. 9, pi. 3, fig. 11. 



Spec. Char. Testa transversa, ovatd, injlatd, inaquilaferali, tenui, postice longiore 

 utrinque convexd ; marline dorsali rotimdato ; apicibm obtusis, depressis. 



Shell transversely ovate, tumid, inequilateral, thin, posterior side the larger, both 

 sides convex ; dorsal margin rounded ; umbones obtuse, depressed. 



Length, f ths. Height, |ths of an inch. 



Locality. Coralline Crag, Sutton, and Gedgrave. Red Crag, Sutton. 



This species is not at all abundant. There are about a dozen specimens in my 

 Cabinet presenting characters that appear of sufficient prominence to entitle it to be 

 considered as different from the preceding one, and a few more particulars may therefore 

 be pointed out to support the opinion. Our shell is more regularly rounded on both 

 sides, and has not the squareness of outline so conspicuously shown in that species, 

 where the dorsal margin forms a straight line, giving a distinct angle on the posterior 

 side, whereas in this one it is eminently rounded, and the Avhole shell is more regularly 

 tumid ; the posterior side is considerably the larger, and the umbones are rather 

 depressed, turning a little towards the anterior, and the ligamental area is smaller than 

 in the preceding species ; the shell is thin, and the muscle marks not very well defined, 

 but where they are seen, they appear to be different in size, the posterior one being the 

 longer, and of a rounded oblong form, and that by the mantle without the least inflec- 

 tion : the teeth are two in each valve, one simple, the other bifid, the posterior one 

 is simple in the left valve, in the right it is anterior; the bifid one is less, and the single 

 one is better defined than in the preceding species ; the whole aspect of the shell is also 

 different, that I have no hesitation in separating the two. This species and rotundata 



19 



