BIVALVIA. 95 



brevi ; basi ellipticd curvatd. Cardo dentibus cardinalibus duobus subtrigonis, et lamina 

 testaced posticd, elongatd, cum dente laterali postico remoto obtuso in utrdque valvd. 

 Impressiones musculares ut in Corbis ; valvium marginibus interni integri. 



Shell destitute of ornament, ovately elongated, rather compressed ; umbones contiguous 

 and depressed, and placed a little anterior to the middle of the valves ; superior or ligamental 

 border lengthened, nearly straight, and sloping obliquely ; ligament external, short, and 

 contained in a groove ; basal margin curved elliptically. Hinge with two cardinal sub- 

 trigonal teeth, a lengthened posterior lamina, and a remote, obtuse, posterior lateral tooth 

 in each valve. Muscular impressions as in Corbis, the anterior impression being small and 

 oval, the posterior larger and more rounded, the inner margins of the valves plain. Casts 

 of a large Inferior Oolite species exhibit an oblique anterior sulcation, which passes down- 

 wards immediately behind the anterior impression, and is obliterated towards the lower 

 border ; this sulcus indicates the presence of an oblique rib upon the interior of each of 

 the valves. The character of the hinge is shown in Tab. XII. fig. 13, 13<z. 



This group of shells, of which the Great Oolite contains a small species, consists of six or 

 more Oolitic species, which all agree in their characteristic features ; their external aspect is 

 sufficiently distinct from the typical group of Corbis, their surface is destitute of ornament, 

 and the greater development of the posterior side indicates a distinction, which is confirmed 

 by an examination of the hinge characters. The anterior lateral tooth is always absent, 

 and the internal ridge, which in the typical form of Corbis descends from it anterior to the 

 impression, passes in our group posterior to the impression, as is clearly shown by the groove 

 in the cast. The stratigraphical position of the known species of this group is as follows. 

 The Inferior Oolite of the Cotteswolds has two species ; our Great Oolite shell is the third ; 

 a large elongated shell in the Coralline Oolite of Malton is the fourth ; the Corbis depressa, 

 Desh., from the department of the Meuse, is the fifth ; and another, probably, is the 

 Psammobia Moreana, Buvig., 'Pal. de la Meuse' Atlas, pi. iv, figs. 8 10 ; the latter form 

 nearly resembling our Great Oolite species. The number of these species, and their general 

 accordance in form, surface, and hinge characters, indicate a distinctness worthy of con- 

 sideration. M. Buvignier, in his description of Corbis depressa, Desh., ' Pal. de la Meuse,' 

 p. 1 2, has, we believe, correctly indicated the natural affinities of this group ; he regards it 

 as establishing a passage between Corbis and Hettangia (Tancredia). Adopting this view, 

 we would likewise place it intermediate to Corbis and the latter genus. 



CORBIS (CORBICELLA) BATHONICA. Tab. XIII, fig. 14. 



Testa ovato-elongatd subcompressd tenui ; umbonibus antemedianis ; latere antico rotunda, 

 postico elongate, subtruncato, angulo obliquo obtuso ; basi curvatd ; lateribus plicis incremenh 

 panels, irregularibw. 



Shell ovately elongated, rather compressed, the test thin ; umbones small, anterior to 



