BIVALVIA. 49 



knotted carinac, the lanceolate, post-ligamcntal space is much lengthened, smooth, and 

 excavated. The costated portion of the shell has the rows at first regular and concentric, 

 with regular, distinct tubercles ; subsequently the costse become more ridge-like and the 

 tubercles less separated ; anteriorly they are small, and the rows arc broken and confused ; 

 posteriorly they are large and more regular, curving upwards slightly, but their extremities 

 are well separated from the marginal carina. 



This is the shell attributed by Messrs. Young and Bird to T. clavellata, and subsequently 

 also by Professor Phillips, Professor Williamson, and Mr. Bean, in their lists of Cornbrash 

 fossils. 



Trigonia signata, Ag., figured in the second part of the Great Oolite Monograph 

 under the name of Ti'igonia decorata, is also an elongated shell, but is destitute of the 

 recurvature of the umbones and of the produced anterior side ; the rows of costae likewise 

 differ ; the posterior portions are not larger than the anterior, and there is wanting that 

 arrest in the continuity of the rows always conspicuous in the Cornbrash shell, and which 

 imparts to the anterior portion of the latter form a broken, irregular character. 

 Trigonia clavellata, Lhwyd, Parkinson, and Sowerby, so abundant in the Lower Cal- 

 careous Grit of England, Prance, and Switzerland, has a much shorter and more convex 

 figure, the umbones are not recurved, features which will suffice to distinguish them 

 irrespective of the ornamentation of the. surface. T. perlata, Ag., and T. Bronnii, Ag., 

 from the same beds, appear to be only varieties of T. clavellata. Trigonia Scarburgensis 

 is also allied to that beautiful and well-known Oxford Clay representative of the ClavellatcB 

 so long procured at Weymouth, and of which a good figure is given in Mr. Damon's 

 'Geology of Weymouth,' Suppl., pi. ii, fig. 3; the latter, in addition to the unusual 

 elongation of its posterior side, has a wide diagonal space, destitute of ornament, separating 

 the posterior extremities of the costse from the marginal carina. 



Geological Position and Locality. Trigonia Scarburgensis is moderately common in 

 the Cornbrash of the Yorkshire coast ; it may also occur in the same rock of the southern 

 counties, but the condition of the specimens is such that it has not been ascertained with 

 any confidence. 



Trigonia Cassiope, B'Orb. Tab. XXXVII, fig. 10. 



Trigonia Cassiope, VOrb. Prodrome de Paleont., I, p. 308. 



Testa ovato-trigona, transverse elongata, subdepressa, costis transversis, sub/wrizontalibus, 

 numerosis, latvigatis, gracilibus curvatis, antice rotundata, postice producta ; area trican- 

 nata, carina marginali et interna crenulata, carina mediana parva ; carinarum intervallo 

 costellis longitudinalibus granosis, confertis, ornatis ; area postica lanceolata, delicate 

 reticulata. 



Shell ovately trigonal, transversely elongated, somewhat depressed ; transverse costse 



7 



