KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3- 91 



the origin of Diceras, which he assumes is to be sought for in the Megalodontidae. 

 This view, as we have said, has been accepted by modern authors; thus Frech 

 (1912) gives a detailed »Entwurf eines Stammbaumes der Megalodontiden», basing 

 on the latest scientific results and completing in a very interesting manner as well 

 as verifying, in the essential, the arguments given by Hoernes. Frech seems, 

 however, to maintain the opinion that the Chamidae have emanated directly from 

 the Megalodontidae, supporting his assumption by the fact that the difference be- 

 twen the two families is the inequilaterality of the former, while the latter is chiefly 

 equivalve, and that there exist forms of Megalodus which apparently mediate the 

 transition by their rather advanced inequivalvity (e. g. M. loczyi). 



This supposition of Frech's may prove to be correct or not — at present 

 nothing can be said about its probability, because the hinge of M. loczyi is still 

 unknown. It shows, however, how closely Chamidae and Megalodontidae are related 

 to each other. If the species mentioned agrees in shape with Chama there are also, 

 on the other hand, examples of a very great similarity in the hinge construction. 

 Thus we find in Physocardia (for example Ph. hornigi Bittner 1912, pl. I, figs. 1 

 —4) a hinge with a thin ridge representing tooth 1 , tooth 2 is complete and so is 

 tooth 3, and 4 is very short; all teeth are placed parallel to the hinge margin, and 

 all these characteristics are common to Chama. A further similarity is to be ob- 

 served in the shape of the anterior muscular impression, which, as Bittner says 

 (p. 10) »bei Megalodon in einer Art engen Spaltes zwischen Schlossplatte und Vorder- 

 rand liegt, während er bei unserer Veczprémer Art eine weite Ausrandung an der- 

 selben Stelle einnimmt». In addition to the equivalvity a difference from Chama 

 is shown by the absence of posterior lateral teeth. 



If the genus Physocardia reminds us of Chama there are also, on the other 

 hand, forms of Megalodontidae which, on account of a strong development of tooth 

 1 and a division of tooth 2, seem to be ancestors of Pseudochama; such a species 

 is, for example, Megalodus co?npressus (ef. Frech 1912, p. 89, fig. 100) and in a 

 higher degree Protodiceras pumilus (Frech 1912, fig. 101). In the last-named spe- 

 cies, of which Boehm (1891) first ga ve a correct description of the hinge and a new 

 opinion of its systematical position, we notice in the left valve a horseshoe-shaped 

 tooth grasping the small anterior one of the right valve, the latter thus being 1 

 and the former 2 with its före and back parts (2 a, 2 b). In this form we find a 

 striking resemblance to Pseudochama and a step towards the hinge type of Diceras 

 just described, and because of this it is considered to be the real ancestor of Diceras, 

 as Boehm was the first to note and Frech has made evident in his phylogenetic 

 scheme of the Megalodus-tvihe. 



O ur investigation has led to the result that the recent Chamidae must be di- 

 vided into two well-defined genera Chama and Pseudochama, of which the former 

 is to be derived from sinistral, the latter from dextral forms of Diceras. On the 

 other hand both genera exhibit a striking similarity to different genera of Megalo- 

 dontidae. In consideration of their great agreement with each other in their ana- 

 tomy it is more probable that both genera have originated from a single and common 



