70 Mr. J. Lycett on Fossil Shells from the 



border ; the costse are crossed by very fine, closely arranged encir- 

 cling striae or lines ; the hinge is without teeth. Height 3 lines, 

 breadth 3 lines. 



The impression figured by M. Agassiz has a much more ob- 

 long or transverse figure, being much lengthened posteriorly ; it 

 is also rather imperfect or truncated at that extremity ; the angle 

 of the costse is placed much more anteriorly than in our species, 

 but inclines like the latter to the antero-ventral border ; the costse 

 are likewise more narrow and numerous. Considering the flat- 

 ness of the valves and their thickness, it would appear that the 

 mollusk was shielded rather than inclosed by them ; the valves 

 would appear to have been open all round except at the ligament ; 

 the character of this latter organ must for the present remain 

 doubtful, as no trace of a lamina for its attachment is visible. 

 Our present imperfect definition of the genus Ptychomya there- 

 fore will be as follows : — Shell equivalve, suborbicular or oblong 

 and transverse, flattened, thick ; umbones small, straight, flat- 

 tened ; hinge-line posteriorly straight or slightly curved ; valves 

 open all round ; surface with numerous curved ribs meeting at 

 an angle, whose apex is directed towards the umbo ; the costse 

 are covered with numerous, closely arranged, concentric striae or 

 lines. Hinge edentulous. 



Of the fossil Myada, Goniomya is the only one which resem- 

 bles it, but in that genus the costse meet at an angle inclined in 

 an opposite direction to Ptychomya ; the surface has similar fine 

 concentric lines, but here the resemblance appears to cease. 



The true position of our genus in the molluscous tribes must 

 therefore remain in abeyance ; the smallness of the object and 

 hardness of the investing stone are formidable obstacles in the 

 way of further information to be obtained from it. 



EXPLANATION OF PLATE II. 



Fig. 1. Turbo elaboratus. 



— 2. Solarium Cotswoldice. 



— 2 a. The same magnified. 



— 3. Chemnitzia gracilis. 



— 4. Gervillia aurita. 



— 5. Opis gibbosus. 



— 5 a. The same magnified. 



— 6. Ptychomya Agassizii. 



— 6 a. The same magnified. 



— 7. Corbis aspera. 



— 8. Tancredia donaciformis. 



— 9. Tancredia extensa. 



— 9 a. Interior of the same. 



— 9 b. Magnified view of the hinge. 



— 10. Tancredia truncata. 



The two latter species pertain exclusively to the Great Oolite. 



