B1VALVIA. 107 



The tenuity of the test is extreme, and the fine radiating lines are usually visible upon 

 the internal casts. The figure varies even more than is usual in the Ceromya. It has 

 some resemblance to Ceromya iriflata, Agassiz, but in that shell the character of the 

 plications and their direction is altogether different, the size, likewise, never attains to that 

 of our species. 



In the greater number of specimens there is a wide depression, which extends from the 

 region of the umbones to the inferior border, giving a compressed aspect to the anterior 

 and inferior portion of the shell. 



It occurs not unfrequently in the upper beds of the Great Ooolite in beds of buff- 

 coloured hard sandstone, situated about 95 feet above the fuller's earth, but always 

 in the form of casts ; in the shelly beds of the formation it occurs very rarely, the test is 

 then preserved, and the valves disunited. 



The form of Ceromya undulata presents the greatest possible contrast to Gresslya, but 

 it is not easy to describe the distinctive features however striking. 



Height, 17 lines; length, 20 lines; diameter through both the valves, 16 lines. 



Locality. Minchinhampton. 



Ceromya plicata, Ay., var. Tab. X, fig. la, b, fig. 2. 



Ceromya plicata, Ag. Etud. Cret. Myes., tab. S d, 1842. 

 Cakdita V-costata, Buchnan. Geol. Chelt., 2d edit., p. 97, 1845. 



Testa ovato-oblonyd, inflatd; umbonibus anticis depressis, involutes ; latere antico 

 brevissimo, ttcmido, truncato ; latere postico lato, aperturd ejusdem magna et elonyatd ; 

 maryine superiore elato ; inferiore subrecto et subundidato ; lateribusfastiyiis lonyitudinalibus 

 crebris, subundtdatis, supeme acutanyulo rejlectis, {alate proyrediente) aliis concentricis 

 decussatis ; lateribus semel in medio sulcis radiantibus obscuris notatis. 



Shell ovately oblong, much inflated about the middle of the valves ; umbones involute, 

 anterior and depressed ; anterior side very short, truncated and tumid ; posterior side wide, 

 its aperture large and lengthened ; superior margin much elevated, and rather compressed ; 

 inferior margin lengthened, nearly straight, and sometimes slightly undulated ; the sides 

 of the valves with closely arranged longitudinal ridges, which slightly undulate, and 

 towards the superior and posterior border are suddenly reflected anteriorly, forming acute 

 angles ; in progress of growth these reflected ridges are nearly effaced, and a second series 

 of concentric ridges are formed, which cross the others obliquely towards the inferior 

 border ; lastly, in adult specimens, there may be distinguished a few obscure radiating 

 sulcations about the middle of the valves. This shell, in the young condition, is a pretty 

 species ; the longitudinal ridges are very distinct, and their V-hke angle towards the 

 superior border is clearly defined ; in adult shells the figure is more ventricose, the superior 

 angle formed by the ridges is nearly effaced ; the second, or concentric series of ridges, are 

 formed, and some few radiating sulcations may be traced. 



