210 CARBONIFEROUS LAMELLIBRANCHIATA. 



umbo were not always characteristic of the various forms described as Schizodus, 

 and suggests that probably many species have been erroneously classed in this 

 genus. Von Wohrman sums up his remarks by stating that it is very question- 

 able whether the genus Schizodus can be retained under the circumstances. 

 Waagen, who admits both Schizodus and Myophoria, says (op. sup. cit., p. 232), 

 " Myophoria is, indeed, very nearly allied to Schizodus ; the cardinal teeth in 

 both valves are perfectly identical, and the only point of difference consists in a 

 more or less strongly developed internal ridge surrounding the anterior muscular 

 impression at its upper extremity, and extending from them a short way up 

 towards the apex of the valves. Also in Schizodus a certain tendency exists to 

 develop internal ridges inside the valves, but they are mostly posterior." This 

 author traces the descent of Myophoria through Schizodus and through Curto- 

 notus to Pseudaxinus, with its thin shell and slightly developed cardinal apparatus. 



Waagen shows that in the Salt Range fossils there are certain forms which 

 are intermediate in character between Schizodus and Myophoria, which are 

 themselves only separated by very minor differences. 



Beushausen ("Die Lamellibranchiaten des Rhein'schen Devon," ' Abhandlung. 

 Preuss. Geol. Lands./ neue Folge, Heft 17, 1895) refers the Trigonida? of the 

 Devonian to Myophoria, and agrees with Freeh that these Devonian shells show a 

 closer resemblance to Myophoria than to the Permian Schizodus. From this it 

 would appear that Beushausen admitted both genera. 



Fleming in 1828 ('British Animals,' p. 426) described a new shell from shale 

 connected with Carboniferous Limestone as Corbula limosa, which may perhaps 

 have been a specimen of one of the species of Schizodus ; but as no figures were 

 given, and the description is very meagre, we cannot be at all certain of what 

 shell is referred to. The description is " transversely subtriangular and longi- 

 tudinally heart-shaped, beaks gibbous, surface slightly striated by layers of growth, 

 shell thin." 



De Koninck in 1885 erected the genus Protoschizodus for certain shells which 

 had been referred to Schizodus by King and others ; this was on account of the 

 marked difference in the hinge-characters, the left valve having two cardinal 

 teeth, the right valve one. The general shape of the shell, too, differs from that 

 of Schizodus, being more regularly triangular and compressed. 



Many of the British species hitherto placed under Schizodus belong to the 

 Protoschizodus of de Koninck, and I have only retained four species in the former 

 genus. The distribution and the nature of the deposits in which these four species 

 are found are of interest. Never found in pure limestone, they all occur in shales 

 or impure concretionary or argillaceous limestones ; one, 8. Pentlandicus, seems to 

 be peculiar to the Calciferous Sandstone series. S. axiniformis has the greatest 

 vertical and horizontal range, being found in the Redesdale Ironstone and in the 



