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 I. minima, Damon (' Geol. 

 Weymouth,' SuppL, pi. 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. The 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. 



ISOCARDIA TENERA. Tab. XXXVIII, figs. 5, 5 a, 5 b. 



ISOCARDIA TENEUA. 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 lamina of the test has 

 decomposed, the striations are decussated by others radiating from the umbones, as is also 

 seen in Isocardia nitida; these radiating striations belong only to the inner layer of the 

 test. 



ISOCARDIA NITIDA, Phil. Tab. XXXVIII, figs. 6, 6 , G b. 



ISOCARUIA NITIDA, Phil. Geol. York., i, pi. 9, fig. 10. 



TIUANGULARIS, Bean. Mag. Nat. Hist., 1839, p. 60, fig. 20.. 

 NITIDA et I. TRIANGULARIS, Morrii. Catal., 1854, p. 204. 



Testa crassa, nitida, injlata, ovato-trigona, umbonibus mediants, altis, acuminatis, sub- 

 involutis, laicre postico angulo obliquo acuto et area postico subconcavo; basi postice sinuato ; 

 valvis striis rcyularibim, longitudinalibus, crcbris ; nucleo lefvi. 



Shell thick, shining, moderately inflated, ovately trigonal ; umbones mesial, elevated, 

 acuminated, subinvolute ; the posterior side with an oblique, acute angle, which separates 



