BIVALVIA. 121 



horizontal costse only exist upon the umbo. Unfortunately our specimen has no portion 

 of the granulated outer surface preserved. 



Length, 19 lines ; height, 12 lines ; diameter through both the valves, about 8 lines. 



Locality. — Blisworth, Northamptonshire. 



PflOLADOMYA, Sow. 



Shell thin, inequilateral, ventricose, oval or oblong ; the borders of the valves more or 

 less gaping, especially at the posterior extremity ; the umbones are large, contiguous, the 

 apex of the one slightly impressing the other ; the ligament external, and placed in an oval 

 depression, the surface is ornamented with costag radiating from the umbones, which are 

 regular and equal or irregular and unequal, smooth and rounded, or deeply notched and 

 nodulous ; the entire surface has concentric plications which vary in their regularity, 

 size, and prominence. The hinge is without teeth, but has an elongated lamina situated 

 beneath the ligament. 



The costse are very commonly more numerous and prominent in the right valve than 

 in the left. 



The muscular impressions are faintly marked and cannot usually be distinguished ; 

 the anterior impression is pyriform and elongated upwards towards the umbones, the 

 posterior muscle is rounded, the syphonal scar has a considerable flexure. 



Pholadomya acuticosta, Sow. Tab. XIII, fig. 13. 



Pholadomya acuticosta, Sow. Min. Con., t. 546, f. 1, 2. 



Testa ovato-elonyatd ; umbonibus crassis, antemedianis, latere antico brevi rotundato, 

 posteriore producto angustato, costis elatis acutis, anticis magnis remotiusculis et 

 irregularibus ; posticis ?iumerosis crebris et tenuibus ; striis concentricis decussatis. 



Shell ovately elongated ; umbones thick, placed anterior to the middle of the valves ; 

 anterior side short and rounded ; posterior side more produced and narrow; costse elevated, 

 acute ; the anterior costse large, rather remote, unequal, and placed at irregular intervals ; 

 the costse posteriorly are less elevated, numerous, and very closely arranged, gradually 

 decreasing in distinctness towards the posterior extremity of the shell ; the costae are 

 decussated by concentric striatums. 



Our species is distinct from Pholadomya acuticosta, Rcemer, tab. ix, fig. 15 ; and from 

 Goldfuss, tab. cxxxvii, fig. 4 ; these, and likewise P. multicostata, Agassiz, tab. ii, figs. 3, 4, 

 have the anterior costse regular and less prominent than in our species ; the P. multicostata 

 varies very considerably in its length, but our species is nearly uniform in figure. 



Localities. — The upper beds of the Great Oolite, near Minchinhampton ; the slate of 



Stonesfield. 



16 



