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 Ornes (Mense), and Launoy (Ardennes), gave them to the public in the ‘ Bulletin 
of the Geological Society of France,’ sér. 2, t. 8, p. 353, under the new generic 
designation of Zsodonta. It is to the researches of M. Buvignier, therefore, that we are 
indebted for a full and accurate description of Sowerbya. 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, S. 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 subzequivalve 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, Phi/., sp. Tab. XXXV, figs. 3, 3a, 34. % 
CucULLHA TRIANGULARIS, Phil. Geol. York., i, pl. 3, fig. 30. 
ARCA TRIANGULARIS, D’Orb. Prodr., i, p. 369. 
CUCULLEA TRIANGULARIS, Mor. Catal., 1854, p. 197. 
Testa transverse, oblonga, inflata, subequilatera, postice oblique carinata, umbonibus 
parvis postero-medianis, margine inferiore angulo formante ; superficie plicis longitudi- 
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 
