BIVALVIA. 59 



with which this species has been figured and described, the foregoing definition will be 

 found to differ from all which have previously been given ; it is founded, however, upon 

 observation of the form in its varieties and stages of growth without stint of examples. 

 It is distinguished from other allied costated species, by characters which are chiefly 

 supplied by the posterior slope, and which are constant and of importance. When from 

 six to eight costa3 have been perfected, the marginal carina acquires large denticulations, 

 and subsequently continues to be indented transversely. The distinctly elevated median 

 carina and finely reticulated surface of the left valve are very different from the correspond- 

 ing parts of the right valve, the area of which has in its middle a longitudinal divisional 

 line which separates the surface into two portions, the posterior portion being more 

 depressed than the other; at first, there appears a kind of median carina, which subse- 

 quently is not to be distinguished from the other plications ; these large plications do not 

 occupy the entire surface of the area, but have between them, and more especially 

 separating them from the marginal carina, wide and depressed interstitial spaces. 

 Goldfuss states, that the apex of the right valve is more recurved, or advances before the 

 other ; this feature has occasionally been observed in specimens from the Cotswolds, it may 

 therefore be regarded not as an accidental but as an occasional feature, which certainly 

 is absent in the majority of specimens. Neither is this character altogether peculiar to the 

 present species of Trigonia. A rigid comparison of specimens proves that the minute 

 Trigonia pullus of the 'Mineral Conchology' from Ancliff, is only the germ of Trigonia 

 costata, not of the typical large Inferior Oolite shell, but of a much smaller variety, which 

 is abundant in the Great Oolite ; adult specimens of this variety, which may be called 

 pullus, have an equal number of costae with the typical form, but the figure is less convex ; 

 the anterior border is not truncated, both that and the inferior border being regularly 

 rounded. The linear dimensions never attain to half of the large Inferior Oolite form, an 

 inconsiderable number only exceed an inch in length, but specimens of half an inch, or 

 even less, are much more abundant. The peculiar features of the cardinal area above 

 described are persistent in all the varieties of the species, and furnish a ready means 

 of distinguishing it from allied costated forms, such as Trigonia similis of Bronn, 

 T. Meriani, monilifera, denticulata, papillata, and suprajurensis of Agassiz ; T. costata 

 of Pusch, ' Polens Palseont.,' taf. vii, figs. 1 , 2, is regarded by Agassiz as a distinct species, for 

 which he proposes the specific name of sonata. Trigonia costata would appear to have 

 very frequently been confounded with an abundant Kimmeridge clay species, but in the 

 latter shell the area is alike in both valves, the marginal carina has not large denticu- 

 lations, the general form is more elongated, the umbones much less recurved, the marginal 

 carina is nearly straight, and the costse are much more oblique. In the Mihchinhampton 

 district the pullus variety of T. costata is exceedingly abundant, surpassing in numbers 

 those of the other Trigonias combined ; the valves are usually disunited, and internal 

 casts are never obtained ; a length of :20 lines upon the marginal carina appears to be its 

 utmost limit in size. 



