66 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



It appears from the above quotation that M. D'Orbigny was acquainted only with 

 the hinge of the right valve of his Sowerbya crassa, upon which species the genus was 

 founded, and that he mistook the mesial dental pit for a fosse destined to receive an 

 internal ligament. In 1851, M. Buvignier having worked out the details of the generic 

 characters from specimens obtained in the upper ferruginous Oolite of the Oxfordian 

 strata of Ones (Mense), and Launoy (Ardennes), gave them to the public in the ' Bulletin 

 of the Geological Society of France,' ser. 2, t. 8, p. 353, under the new generic 

 designation of Isodonta. It is to the researches of M. Buvignier, therefore, that we are 

 indebted for a full and accurate description of Soioerbya. The same author states that 

 M. Terquem has discovered one nearly allied to the typical form in the Bradfordian beds 

 of the Mozelle. 



The Jurassic rocks of England contain upwards of five species of Sowerbya : 1, 

 S. triangularis, from the Oxfordian and Lower Oolites of Yorkshire ; 2, 8. Woodwardi, 

 from the Great Oolite of the Minchinhampton district ; 3, a small abruptly truncated 

 species from the Coral Rag of Yorkshire and Oxfordshire ; 4, a small subaequivalve shell, 

 with a posterior strongly marked oblique angle from the Coral Rag of Bullingdon ; 5, an 

 internal cast of a large species determined by Mr. Woodward, and figured by Mr. Damon 

 in his ' Geology of Weymouth,' from the Portland Oolite, under the name of S. Dukei. 



SOWERBYA TRIANGULARIS, Phil., sp. Tab. XXXV, figs. 3, 3 a, 3 b. 



CUCULL/EA TRIANOULARIS, Phil. Geol. York., i, pi. 3, fig. 30. 

 ARCA TRIANGULABIS, D'Orb. Prodr., i, p. 369. 

 CUCULLJBA TRIANGULARIS, Mor. C.itdl., 1854, p. 197. 



Testa transverse, ollonga, injlata, sulaquilatera, postice oblique carinata, wmbonibus 

 parvis poster o-medianis, margine inferiore angulo formante ; superficie plicis lonffitudi- 

 nalibus paucis magnis et striis longitudinalibus subtillissimis ornata. 



Shell transverse, oblong, inflated, slightly inaequilateral ; the posterior side the shorter, 

 with a posterior oblique angle, separating a posterior slightly excavated surface which 

 terminates downwards in a conspicuous angle ; the anterior side is produced and curved 

 elliptically ; the umbones are placed a little posterior to the middle of the valves ; they are 

 small and contiguous. The surface has one or two large folds of growth, and is orna- 

 mented with longitudinal, regular, closely arranged striations, which disappear upon the 

 posterior excavated slope. 



The height is about equal to the diameter through both the valves, and to three fifths 

 of the length. 



The species exhibits much variability in the general figure, in the degree of convexity, 

 in the prominence of the posterior angle, and in the length ; differences which are not 

 limited to a single formation or locality, as it occurs in the Yorkshire Oolites in the 



