408 Mr. J. Lycett on fossil Shells from the 



species they may be expected to occur over large areas ; already 

 they are known in the lower oolitic system of Normandy. 



The generic appellation is derived from the name of Sir Thos. 

 Tancred, Bart., the founder of the Cotswold Naturalists' Club. 



The descriptions of two species will be found in the notes to 

 the tables of Inferior Oolite shells ; those of the Great Oolite are 

 deferred to the monograph upon that subject. 



Note on No. 199, Ptychomya ? Agassizii. 



At pi. 11. f. 3, 4 of the 'Etudes critiques sur les Mollusques 

 fossiles/ by M. L. Agassiz, is an imperfect impression of an ob- 

 long flattened bivalve shell to which is aflixcd the new generic 

 appellation Ptychomya, but no account is given of the locality or 

 geological formation to which it belongs ; the figure is founded 

 upon a single impression. M. Agassiz has not ventured to de- 

 fine the genus, and in his introduction mentions that M. D'Or- 

 bigny considers it to be a Crassatella, to which genus M. Agassiz 

 remarks it has no external resemblance. 



Having long possessed specimens of a small shell which ex- 

 hibits the external characters of Ptychomya, and as two of the 

 specimens are in a condition nearly perfect, I have ventured to 

 record the little information thus acquired with the impression, 

 that although meagre and imperfect, it should not be withheld 

 when the object of investigation is obscure or unknown; never- 

 theless the present note would not have appeared but for the 

 necessity of affixing a generic name in my Tables to the little 

 shell in question. The high degree of critical acumen displayed 

 by the talented author of the ' Etudes,' together with the just 

 confidence which he shows in the accuracy of his own observa- 

 tions and deductions when controverted by others, rendered it 

 probable that the generic value which he had claimed for this 

 obscure form would eventually be found to be justified; the pre- 

 sent species therefore became an object of interest upon the dis- 

 covery that it could scarcely with propriety be assigned to any 

 other known genus. 



Ptychomya? Agassizii. PL XL fig. 6. 

 Figure suborbicular and flattened ; umbones straight, small, 

 pointed and mesial ; the substance of the shell thick ; the lunule 

 indistinct or very slightly excavated; the hinge-line posteriorly 

 straight or slightly curved and sloping obliquely; the ventral 

 border rounded, the surface with about fourteen rounded, broad 

 but depressed costse, which are curved upwards and meet the 

 costse of the opposite side upon the middle of the shell forming 

 an angle, the points of junction of the several costse being upon 

 a line passing obliquely from the umbo to the antero-ventral 



