BIVALVIA. 85 
they 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-ligamental 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 coste, are so 
faintly marked that they are only visible near to the umbones. The very delicate, 
radiating little rbs and the nearly equally faintly marked longitudinal plications will 
usually serve to distinguish it from allied forms of the genus when combined with the 
elongated figure. In the Pholodomye 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—15—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 
Gryphea sublobata. 
PHouapomya Puinirrsu, Phil, sp. Tab. XLII, figs. 2, 2 a. 
PHotapomya Mourcuisont, Phil. Geol. York., i. pl. 7, fig. 9, non Sow. 
—_ Puiuuirsi, Morris. Cat. Brit. Foss., p. 221 
Testa ovato-cordata, inflatra, 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 rugee, which are not very promi- 
nent, and only slightly indent the narrow, perpendicular costa, 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 cost are 
symmetrical, and descend almost perpendicularly to the lower border, leaving a con- 
siderable space upon the posterior side of the valves destitute of costa. 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 Jess 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. 
