BIVALVIA. 87 



umbones more obtuse, and the general figure less inflated, than obtains in the greater 

 number of those from the ]\Iinchinhampton 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. 



Pholaihmya delioidea is remarkable for the large, prominent, and slightly indented 

 costae, usually seven in number, of which the two posterior ones arc much less conspicuous, 

 and are usually evanescent towards the low^er border. 



The frequent truncation of the anterior side in Fltoladomya, 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. deltoidea, 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 attahiing to the usual dimensions of the species j 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 pointed 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. 



Pholadomya lyrata. Sow. Tab. XLIII, figs. 3, 3 a. 



Cardita? lirata, 8ow. Min. Con., t. 197, fig. 3, 

 Pholadomya lyrata, Sow. Ibid., p. 220. 



— — Oppel. Juraformation, p. 482. 



Testa ohovato-trigona, ventricosa, umbonibus elevatis antero-medianis, latere antico 

 truncato, postico oblique declivi, costis 9 — 10, tertia major a, carinam efformante, costis 

 aliis approximatis, plicis magn'is concentricis regularibus indentatis ; apertura j^ostica 

 angusta, elongata. 



The figure is nearly that of a cone, compressed laterally ; the umbones arc 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, carinated 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 



