BIVALVIA. 69 



In some of my specimens from the Coralline Crag, where the two valves are in 

 their natural state a very perceptible difference may be observed, not only in one 

 valve having a greater tumidity than the other, but the inflated valve has also a larger 

 diameter. Perhaps the ovarium in these specimens occupied a position not quite 

 central, thereby giving a little inequality to the valves. 



There are nodules of indurated sandstone in my cabinet, which contain casts of 

 what appear to be the interior of this species, arid also those of Isocardia cor. where the 

 shell has been absorbed or abstracted ; these nodules were obtained on 'the beaches of 

 Walton Naze and Felixstow, and were in all probability washed out of the Red Crag. 



Limopsis, Sassi. 1827. 



Arca (spec). Brocchi. 

 Trigonoccelius. Nyst et Galeotti, 1835. 

 Limnopsis. Gray, 1840. 

 Pectunculina. jyOrb, 1844. 

 Crenella. Herrmansen, 1846. 



Generic Character. Shell orbicular or obliquely ovate, convex or lenticular, equi- 

 valved, subequilateral, and closed. Hinge composed of numerous teeth, arranged in 

 a more or less curvilinear direction, projecting and interlocking. Umbones distant. 

 Cardinal area large and external, divided by a triangular fossette immediately beneath 

 the umbo. Impression of the mantle entire, or without a sinus ; those by the adductors 

 subovate, and deeply impressed. 



ANIMAL UNKNOWN. 



The characters by which this Genus is distinguished from the preceding one is 

 the triangular fossette in the centre of the ligamental area, separating the cartilage 

 from the ligament ; first proposed as of generic importance by Sassi, in 1 827, 

 according to Bronn, and his name has priority over that by MM. Nyst and Galeotti, 

 which bears a date several years later. This peculiar character of the hinge 

 was observed and pointed out by Brocchi in 1814, but of course considered by him 

 as of specific value only, his shell being placed in the genus Arca. The separation 

 of the ligament into two distinct portions, although both of these are placed exterior 

 to the hinge line, appears equivalent to the otherwise more general distinction of this 

 ligature, one portion being within the hinge line, while the other is on the outside. 

 Eighteen species are enumerated by M. Nyst, one of which is recent from the 

 Red Sea. 



