Inferior Oolite in Gloucestershire. 409 



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 j 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 Myadce, 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 XL 



T^g. \. Turbo elaboratus. 



— 2. Solarium Cotswoldics. 



— 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. 



