83 Mr. J. Lycett on the Hinge of the Fossil Genits Platymya. 



remaining one to the middle division of the Oolitic system. The 

 present species has claims upon our notice beyond that of a new 

 species merely, inasmuch as it is the first English recorded ex- 

 ample of the genus, — the first which has been identified in the 

 lower division of the Oolitic system ; and lastly, it has the im- 

 portant and novel advantage of having its test preserved and the 

 character of its hinge clearly exposed. Although six years have 

 elapsed since the publication of the ' Etudes Critiques,^ the state 

 of uncertainty in which the hinges of several of the genera 

 therein described were left by its distinguished author has not 

 hitherto been removed ; our own literature more especially is de- 

 ficient in information relating to the extensive family of fossil 

 Myadse : these circumstances it is tnisted will be deemed a suf- 

 ficient excuse for presenting a brief description of the genus Pla- 

 tymya translated from the before-mentioned work of M. Agassiz. 



" The Platymyas are near to the Arcomyas in their form and 

 general physiognomy, but are distinguished by a general flatness 

 of the valves, by the nearly median position of the umbones, which 

 are very depressed, by the extremities being much developed 

 and very large. The two exti'emities gape much, more especially 

 the posterior one. The cardinal area is much less characterized 

 than in the Arcomyas; the marginal keel which separates the 

 area from the sides is not very distinct, and consequently is of 

 little assistance in the determination of species. The ridges or 

 folds of the sides are usually distinct, concentric and well marked 

 upon the anterior side, but more indistinct and irregular upon 

 the posterior. The lines of growth are not usually observed 

 upon the exterior of the moulds, neither are they visible upon 

 the internal moulds. Hinge unknown. The position of the 

 umbones will always distinguish it from Gresslya, Homomya and 

 Myopsis ; Mactromya is usually more short and convex.^' 



It will be perceived from the above extract, that a comparison 

 of our new shell with the several species of Platymya must de- 

 pend upon the external form only, inasmuch as no direct know- 

 ledge of the hinge has heretofore been obtained, and the remarks 

 of M. D'Orbigny are based solely upon impressions in the 

 moulds. Whatever value however may be attributed to impres- 

 sions in moulds must yield to a disclosure of the hinge itself, 

 and in the present instance this direct evidence is combined with 

 a shell whose external characters agree with those of Platymya, 

 and cannot with propriety be referred to any other known genus. 



In two instances we have succeeded in exposing the hinge in 

 each valve, and our definition of Platymya, derived from these 

 examples, will be as follows : — 



Shell thin, nearly equivalve, transverse, compressed ; umbones 

 small, depressed, contiguous, submesial ; cardinal area indistinct. 



