. MYOPHORIA. 95 



beds. It is also to be observed that one of them so exactly resembles Curtonotus 

 elegans, Salter, as to render it very probable that it is only a variety of the same 

 species. 



We next come to some American forms which are strikingly like our sup- 

 posed type-specimen, but which appear generally to differ from our fossils by having 

 more elevated umbones. This seems probably to be due to the American fossils 

 retaining their shells, while our specimens are all in the nature of casts. 



8cMzodus quadrangularis, Hall, and Sch. Chemimgensis, Conrad, appear 

 identical with each other and with the present shell, except that they are about 

 half as large again — a fact which could hardly give them more than varietal rank, 

 as both the English and American shells vary considerably in size. Schizodus 

 adpressus, Conrad, is regarded by Hall as " probably only a variety of Sch. 

 Chemung ensis which lived under different conditions." Sch. sequalis, Hall, is 

 separated by him from Sch. Chemimgensis, var. quadrangidaris, as having " the 

 base more broadly rounded and the anterior portion more expanded below ; " 

 but, if he be correct in making the latter form a variety of Sch. Ghemungensis, 

 it is difficult to see why the former also should not be included in it, and that 

 form is approached by the specimen from West Angle. As given by Whiteaves,^ 

 however, Sch. Chemung ensis is wider and more rounded. 



On the whole there seems reason for regarding these various forms as not 

 more than varieties of one variable species. 



Schizodus rhomheus, Hall, may also possibly be a variety ; but, though 

 equalling the English shells in size, it seems essentially to differ from the adjacent 

 forms in being longer, and in having less anterior inflation. 



Axinus orhicidaris, M'Coy, has much the appearance of being the young or 

 dwarfed form. 



Affinities. — C. centralis, Salter,^ seems distinguishable by its larger and more 

 terminal muscle-scars, its more central umbo, and its more oval form, without 

 signs of posterior truncation. 



Schizodus ohrotundatns, Beushausen,^ appears to have a slightly larger hinge, 

 and not to be flattened and angulated on the posterior side. 



Axinus obliquus, M'Coy,* seems to have a much less inflated anterior side. 



1 1891, Whiteaves, ' Contr. Canad. Pal.,' vol. i, pt. 3, p. 241, pi. xxx, figs. 5, 5 a. 



2 1863, Salter, 'Quart. Journ. Geol. Soc.,' vol. xix, p. 495, figs. 4«, b. 



^ 1884, Beushausen, ' AbhancU. Geol. Specialk. Preuss.,' Band vi, pt. 1, p. 95, pi. vi, figs. 7, 8. 

 * 1844, M'Coy, ' Syn. Carb. Foss. Ireland,' p. 64, pi. v, fig. 29. 



