BIVALVIA. 85 



ihey are slightly impressed by the decussating plications, and disappear before reaching 

 the lower border; the apertures, both posterior and antero-inferior, are narrow, and not 

 much lengthened ; the post-ligaraental area is wide, and bounded by a distinct elevation 

 upon each side. 



The test is thin, and is sometimes preserved, the characters of the surface being very 

 well shown upon the casts ; in many specimens the radiating lines, or little costa3, are so 

 faintly marked that they are only visible near to the umbones. The very delicate, 

 radiating little ribs and the nearly equally faintly marked longitudinal pUcations will 

 usually serve to distinguish it from allied forms of the genus when combined with the 

 elongated figure. In the Pliolodomyae the relative measurements of the parts are little to 

 be depended upon ; but in the Cornbrash specimen figured, the diameter, the height, 

 and the length, are as 1 — 1^ — 2. Occasionally the length has a greater proportion. 



Geological Positions and Localities. It is somewhat rare in the Cornbrash of the 

 coast of Yorkshire, but it is common in the Inferior Oolite of the Cotteswold Hills, its 

 position being the stratum with Conchifera immediately underlying the bed with 

 Gryplieea suhlohata, 



Pholadomya Phillipsii, Phil., sp. Tab. XLII, figs. 2, 2 a. 



Pholabomya Murchisoni, Fhil. Geol. York., i. pi. 7, fig. 9, non Sow. 

 — Phillipsii, Morris. Cat. Brit. Foss., p. 221. 



Testa ovafo-cordafh, injlatra, umbonibus magnis elevatis, antice brevissima truncata, 

 postice producta, valde hiante ; lateribus rugis irregularibus numerosis, leviter impressis, 

 costisque (7 — 8) perpendiculariter, angustis ; costa secunda majora. 



Shell ovately cordate, much inflated ; umbones large, anterior, elevated, but obtuse ; 

 anterior side short and truncated ; posterior side produced, its superior border concave, 

 with a lengthened, large aperture, which extends upwards even to the ligament ; the sides 

 of the valves have very numerous, irregular, longitudinal rugae, which are not very promi- 

 nent, and only slightly indent the narrow, perpendicular costse, of which there are seven or 

 eight ; the second costa is much larger than the others, and is more remotely placed, 

 imparting a degree of angularity to the anterior side of the shell ; the other costse are 

 symmetrica], and descend almost perpendicularly to the lower border, leaving a con- 

 siderable space upon the posterior side of the valves destitute of costoe. The young shell 

 is much less inflated, and more produced upon the posterior side, the aperture at that part 

 being, in proportion, more narrow ; the second costa has very little more prominence than 

 the others, so that the anterior side has less angularity and its border is more rounded 

 than in the adult form. In old specimens the height and convexity of the valves are 

 nearly equal, the length being a little more considerable ; in young shells, not exceeding an 

 inch and a half in length, the convexity is one third less. 



