BIVALVIA. | 57 
and is probably also distinct; it is from a lower geological position, associated with 
Trigonia signata, Ag., and other Inferior Oolite fossils of that stage. 
Another shell erroneously attributed to our species is Z minima, Damon (‘ Geol. 
Weymouth,’ Suppl., pl. iv, fig. 7), from the Coral Rag of Weymouth; it is much more 
oblique, with produced umbones. 
The figures given in the ‘ Mineral Conchology’ and in the ‘ Geology of Yorkshire’ may 
each be objected to for the great prominence of the umbones and the large excavation of 
the lunule. he Yorkshire examples, which are very well preserved, differ somewhat from 
each other in the degree of their obliquity ; some have a slightly defined, oblique, posterior 
angle. The height and length are usually equal, the diameter through the valves being 
one fourth less. 
Geological Position and Locality. The Cornbrash of the Yorkshire coast, in which 
it is not uncommon. 
Tsocarpia TENERA. Tab. XXXVIII, figs. 5, 5 a, 54. 
IsOCARDIA TENERA. Gr. Ool. Monog., t. 7, fig. 1, part 2, p. 66. 
— — Ibid., t. 38, fig. 5, Supplement. 
As the figures given in the former portion of this Monograph represent a specimen 
deprived of the test, a fine example in a perfect condition is now given, together with a 
magnified figure of the ornamentation of the surface. The Cornbrash and the Lower 
Calcareous Grit of Yorkshire yield specimens with the test very beautifully preserved, 
brown and shining. One from the former rock has been selected ; the valves have delicate, 
regular, concentric striations ; and when a portion of the external Jamina of the test has 
decomposed, the striations are decussated by others radiating from the umbones, as is also 
seen in /socardia nitida ; these radiating striations belong only to the inner layer of the 
test. 
Isocarp1a nit1DA, Phil. Tab. XXXVIII,' figs. 6, 6a, 64. 
Isocarpia niTIDA, Phil, Geol. York., i, pl. 9, fig. 10. 
— TRIANGULARIS, Bean. Mag. Nat. Hist., 1839, p. 60, fig. 20. 
— nivipa et I, TRIANGULARIS, Morris. Catal., 1854, p. 204. 
Testa crassa, nitida, inflata, ovato-trigona, umbonibus medianis, altis, acuminatis, sub- 
tnvolutis, latere postico angulo obliquo acuto et area postico subconcavo,; basi postice sinuato ; 
valvis striis regularibus, longitudinalibus, crebris ; nucleo levi. 
Shell thick, shining, moderately inflated, ovately trigonal; umbones mesial, elevated, 
acuminated, subinvolute ; the posterior side with an oblique, acute angle, which separates 
8 
