Inferior Oolite in Gloucestershire. 421 



148. Mytilus subrectus ; elongated, smooth, slightly oblique, ante- 

 rior border straight, the two extremities of the shell attenuated ; 

 hinge-line lengthened, straight. 



1 50. Mytilus crenatus ; thick, oblique, convex ; length less than 

 twice the width ; striae regular, concentric, deeply grooved upon the 

 back ; a single depression or fold (its edge acute) passes obliquely 

 from the umbo to the antero-ventral border ; hinge straight, mode- 

 rately long. 



151. Breissena lunularis ; smooth, anterior border straight or 

 slightly concave, posterior side curved, umbones pointed, terminal, 

 longitudinal ridge acute, anterior diameter through both valves equal 

 to the breadth. 



152. Gervillia tortuosa ; this shell, the GastrochcBna tortuosa of 

 Phillips, belongs to a very remarkable section of the Gervillice, of 

 which 6r. Hartmanni and G. Monotis are likewise examples ; they are 

 tortuous, very inequivalve, the right valve being more or less concave, 

 its borders fitting closely to the undulations of the convex valve. 



154. Gervillia aurita ; equivalve, smooth, very oblique, both the 

 auricles very much extended and acuminated, the entire figure being 

 very slender. 



156. Gervillia IcBvigata ; smooth, very oblique; anterior auricle 

 produced and rather pointed, posterior moderately large,"^^; left valve 

 convex, right nearly flat. This shell is more oblique than G. costa- 

 tula, and wants the ribs of that species. 



159. Pteroperna ; a group of shells proposed to be separated from 

 the Gervillice and Pernce, to both of which genera they possess affi- 

 nities, combined with the external form of Avicula. A species very 

 nearly allied to our P. gibbosa is abundant in the Great Oolite ; our 

 shell however is more convex and oblique. 



161. Pinna hast at a ; spear-shaped, compressed, lines of growth 

 waved and strongly marked ; lines longitudinal, delicate, waved and 

 closely arranged, crossed by others more distinct. 



164. Hiatella interlineata \ subquadrate ; anterior side rounded, 

 posterior straight and truncated ; costae transverse, large, few, ele- 

 vated, forming an angle upon the back of the shell ; the interstitial 

 spaces have numerous encircling very fine lines. 



169. Opis angustatus \ narrow, elongated, extremely convex, for- 

 nicated ; lunule large, posterior depression cordate, large and deep, 

 with a wide longitudinal sulcus posterior to the carina ; ribs concen- 

 tric, closely arranged, posterior side with densely arranged fine trans- 

 verse lines ; carina moderately elevated and impressed by the costse ; 

 umbones narrow and incurved. 



171. Opis gibbosus ; subtrigonal or cordate, convex, anterior side 

 and base nearly straight ; umbones large, dorsal carina obtuse and 

 scarcely elevated ; costse large, both upon the anterior and posterior 

 side ; lunule cordate and deep, inner margin toothed. The nearly glo- 

 bose form, nearly obsolete carina, and ribs upon the posterior side, se- 

 parate it from contemporaneous species. The size does not usually 

 exceed that of a pea ; with increase of growth it becomes more trigonal. 



1 7Q. Trigonia costatula ; transversely oblong, anterior side trun- 



