BIVALVIA. 49 
knotted carinz, the lanceolate, post-ligamental 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 costee become more ridge-like and the 
tubercles less separated ; anteriorly they are small, and the rows are 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 7. c/avellata, 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 Ziigonia decorata, is also an elongated shell, but is destitute of the 
recurvature of the umbones and of the produced anterior side ; the rows of costz 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, France, 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. 7. perlata, Ag., and 7. Bronnii, Ag., 
from the same beds, appear to be only varieties of 7. clavellata. Trigonta Scarburgensis 
is also allied to that beautiful and well-known Oxford Clay representative of the Clavellate 
so long procured at Weymouth, and of which a good figure is given in Mr. Damon’s 
‘Geology of Weymouth, Suppl., pl. ii, fig. 3; the latter, im addition to the unusual 
elongation of its posterior side, has a wide diagonal space, destitute of ornament, separating 
the posterior extremities of the costze 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. 
Triconia Cassiopz, D’Ord. Tab. XX XVII, fig. 10. 
TRIGONIA Casstopr, D’Ord. Prodrome de Paléont., 1, p. 308. 
Testa ovato-trigona, transverse elongata, subdepressa, costis transversis, subhorizontalibus, 
numerosis, levigatis, gracilibus curvatis, antice rotundata, postice producta ; area tricari- 
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 costae 
fod 
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