BIVALVIA. 123 



Shell ovately elongated ; convex anteriorly, attenuated posteriorly ; anterior side short, 

 its border rounded ; posterior side lengthened and attenuated, its aperture small ; ligamental 

 border nearly straight, sloping obliquely downwards, lower border curved ; radiating costs: 

 (about 7) distant, equal, spreading over nearly the whole of the shell, and rendered nodulous 

 by sOrne large longitudinal plications ; the costoe of the left valve more prominent than those 

 of the right. The convexity of the valves is considerable towards the anterior side ; the 

 umbones, though large, are but little elevated, and these features, together with the few 

 distant and large knotted costa3, will serve to distinguish it from the Inferior Oolite 

 Pholadomya ovulum, Ag., and Pholadomya ovalis, Sow., to both of which species it has 

 some affinities ; from the Pholadomya Murchisoni from Srora, it is distinguished by having a 

 more elongated form, and much more distantly arranged costae, so that only a small 

 portion of the test is without them. 



Dimensions. Length, 2 inches; height, 1| inch; diameter through both the valves, 

 1 inch. 



Geological position and localities. We are not aware that this species has occurred 

 except in the Great Oolite of the North of England ; the specimens forwarded to us are 

 from Scarborough, and from the vicinity of Stamford. 



PHOLADOMYA S^EMANNI. Tab. XI, fig. 1, et Tab. XV , fig. 3. 



Testa ovato sub-compressd ; umbonibus elatis magnis ; latereantico brevi, rotunda ; postico 

 sub-compresso, brevi ; aperturd angustd ; costis radiantibus, 7-8 depressis, subrectis subaequali- 

 bus, et remotis ; plicis longitudinalibus impressis. 



Shell ovate, rather compressed ; umbones elevated and large ; anterior side short, 

 rounded, posterior side rather compressed and short, gaping, with a narrow aperture; 

 radiating costa3 7-8 depressed, nearly straight, equal, regular, and remote; decussated but 

 not much impressed by the longitudinal plications. The lateral diameter is somewhat less 

 than the height, and exceeds considerably the diameter through both valves ; but there is 

 some variation in these proportions, the specimens which have the least convexity being 

 usually less regularly ovate and rounded at their borders, so that they might, perhaps, be 

 divided into two varieties. 



From P. solitaria it is distinguished by the compressed posterior extremity, by 

 the smaller convexity of the valves, and by the character of the cost, which are less 

 elevated and diverge so much more considerably that they nearly occupy the surface of the 

 valves. 



Localities. Small openings or pits in the Great Oolite near to its base, and in the 

 vicinity of the village of Avening. Scarborough, in the Great Oolite. 



