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



its superior border having in each valve a narrow elongated 

 groove with an acute edge, as in Mactromya ; both sides of the 

 shell wide, more especially the posterior one, which is truncated ; 

 both extremities gape slightly, more especially the posterior ex- 

 tremity; ventral margin regular, curved moderately and ellip- 

 tically. Hinge plate internally incrassated and lengthened pos- 

 teriorly, having a single small obtuse cardinal tooth in the left 

 valve and a corresponding oval pit in the right valve ; lateral teeth 

 none ; muscular impressions unknown. 



The hinge apparatus maybe regarded as forming an .exception 

 to the usual characters of fossil Myadse, which are for the most 

 part edentulous ; the present form however can only be consi- 

 dered as an aberrant modification of the same kind of hinge : the 

 tooth is small ; it is of an oval figure, its greater length being 

 lateral ; it projects but little, and the opposite corresponding pit 

 consequently is but shallow. This kind of hinge, taken in con- 

 nexion with the other characters of the shell, will be found to 

 remove it from all other genera of the Myadse, both recent and 

 fossil : there is nothing resembling the projecting spoon-shaped 

 process and accessory tricuspid osseous rib supporting an internal 

 ligament, as in Anatina ; on the contrary, there is every reason 

 to believe that the ligament was external and supported upon the 

 lengthened posterior grooves. The delicacy of the test and hard- 

 ness of the matrix have foiled our attempts to expose the mus- 

 cular impressions. 



The tendency of these details then is to support the conclusion 

 of M. Agassiz with regard to the generic value of Platymya, a 

 conclusion at which he arrived from a consideration of certain 

 external characters only; these however constitute a generic 

 entireness upon which he relied with confidence even after a 

 palaeontologist of eminence had pronounced an adverse opinion, 

 and he remained without the means of verifying his inductions 

 by an examination of the hinge. The dental characters however 

 of the several genera of fossil Myadse would seem to be of much 

 less relative importance than they acquire in certain other fami- 

 lies of the Lamellibranchiate Mollusks. In the fossil Myadse 

 the teeth are for the most part absent altogether, the ligamental 

 support being derived from a thickening internally of the poste- 

 rior and superior border, forming a kind of lengthened posterior 

 rib, and it is the only portion of the shell which is not thin and 

 delicate. Without entering into details respecting the hinges of 

 the several genera, it may suffice to mention that Mactromya, 

 Goniomya, Cercomya, Ceromya, Homomya, Myopsis and Ar corny a 

 have all with certain modifications this description of hinge ap- 

 paratus, which should be regarded as of coordinate rank only 

 with other characters which are external and arc connected with 

 the general form and markings of the surface. Platymya has a 



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