J. F. BLAKE ON THE KIMMERIDGE CLAY OP ENGLAND. 229 



beaks ; surface as a whole smooth in appearance ; breadth J in., 

 height \ in. 



The keel is not so strong as in A. rhomboidalis ; and the ornaments 

 are different. 



Rare in Lower Kimmeridge, Market Easen. 



Arca reticulata, n. sp. PI. XII. fig. 11, 



The sides curve to meet the hinge-line, cutting it ultimately 

 nearly at right angles, the general direction making an angle of 

 about 100°. The umbonal region is not bounded by a keel, but is 

 rounded off both before and behind ; a radial rising departs from 

 below the beak, making a small angle with the posterior part of 

 the hinge-line; inferior margin concave in the middle, corre- 

 sponding to a transverse depression which reaches the umbo, di- 

 viding it into two parts ; beaks about f from the posterior end ; 

 space between the beaks zero; hinge-teeth those of the section 

 Macrodon ; ornaments fine transverse ribs becoming broader, with 

 others intercalated, crossed by concentric strise ; breadth twice the 

 height. 



This has the general shape and ornaments of A. cemula, but 

 differs in the radial rising and the division of the umbo. The divi- 

 sion of the umbo also separates it from other similar forms. 



Less rare in Lower Kimmeridge, Market Easen. 



MODIOLA SEMIPLICATA (BuV.). 



I suspect that many of the so-called Mytilus pectinatus belong to 

 this species. M. pectinatus, however is very distinct, being tri- 

 angular, with a broad base, and a true Mytilus. 



Inoceramus expanstts, n. sp. PI. XII. fig. 7. 



Beaks very prominent ; the two sides of the shell meet there at a 

 right angle when not broken. The posterior part is expanded so as 

 to be almost concave ; when this is broken off, the inflated central 

 part looks much more pointed. Ornament very gentle concentric 

 ridges ; length 3 in., breadth 2|. 



e i=zMytilus trapezus (Cont.) ; but it is not a Mytilus. Yery com- 

 mon in the Lower Kimmeridge. Market Easen. 



Perna mytiloides (Lam.). 



The specimens from the Passage-beds are much broader than usual 

 in this species, and may perhaps belong to another. 



AVICTJLA .^DILIGNENSIS, n. Sp. PI. XII. fig. 2. 



Umbo | the length of the hinge from the anterior end, which is 

 gently convex, not reaching far beyond the hinge, posterior end very 

 oblique, much produced, joined to the wing (which it far out- 

 reaches) by a sharp parabolic curve ; wing of moderate length ; orna- 

 ments delicate ribs, with a bold sweep from the umbo, with finer inter- 

 mediate ribs rising at some distance from the apex, and often two still 

 finer ribs on each side, between them and the primary ribs - x breadth 



r2 



