BIVALVIA. 87 
umbones more obtuse, and the general figure less inflated, than obtains in the greater 
number of those from the Minchinhampton district ; and, upon the other hand, all of them 
are less lengthened upon the posterior side than is seen in the figures given by Agassiz 
and by Messrs. Chapuis and Dewalque. 
Pholadomya deltoidea is remarkable for the large, prominent, and slightly indented 
costa, usually seven in number, of which the two posterior ones are much less conspicuous, 
and are usually evanescent towards the lower border. 
The frequent truncation of the anterior side in Pholadomya, and the general distortion 
of the shell which usually accompanies it, is a source of frequent difficulty in the dis- 
crimination of species, and is seen commonly and even usually in P. de/foidea, when 
specimens are collected without selection. The shell in its normal position rested upon 
the compressed anterior side; the general distortion of figure consequent upon it com- 
menced at a very early period in the growth of the mollusc, continued throughout its 
existence, and did not prevent it from attaining to the usual dimensions of the species ; it 
occurs equally in all the other forms assumed by the genus. Where the flattening of the 
anterior side is considerable the umbones become more poimted and prominent, the 
radiating elevations or ribs are directed more obliquely backwards ; in other instances they 
become more closely arranged, or they are waved and irregular. The distortion is not 
limited to specimens connected with any particular kind of lithological condition, as it 
occurs in compact, thin-bedded limestones, in thick beds of soft, sandy marl, or in thick- 
bedded Oolitic limestone; it is also common to find both compressed and uncompressed 
examples in contiguity. 
Puouapomya Lyrata, Sow. Tab. XLII, figs. 3, 3 a. 
Carpita? LiraTA, Sow. Min. Con., t. 197, fig. 3. 
Puotapomya Lyrata, Sow. Ibid., p. 220. 
= — Oppel. Juraformation, p. 482. 
Testa obovato-trigona, ventricosa, umbonibus elevatis antero-medianis, latere antico 
truncato, postico oblique declivi, costis 9—10, tertia majora, carinam efformante, costis 
aliis approvimatis, plicis magnis concentricis regularibus indentatis ; apertura postica 
angusta, elongata. 
The figure is nearly that of a cone, compressed laterally ; the umbones are elevated, 
pointed, and placed a little anterior to the middle of the valves; the steepness and 
straightness of the posterior slope, together with the nearly straight lower border, imparts 
a distinctive character to the shell, irrespective of the large, carimated third costa, which is 
so much more conspicuous than the others that it forms a kind of keel or angle upon the 
anterior side; it descends to the lower border without curvature, but is directed slightly 
