140 PALAEONTOLOGY OF NEW YORK. 



muscular impressions in that genus lying upon the median thickening but in 

 front of it in the others. The presence of a median septum in both Obolus 

 and Trematis adds to their similarity. The large median scars in Trematis, 

 however, are composite, and thus indicate an important difference in the char- 

 acter of the muscular anatomy of the two genera. In position, if not in func- 

 tion, the anterior and median members of the great central scars in Trematis 

 correspond with the anterior laterals and centrals in the brachial valve of 



LiNGULA. 



In ScHizoTRETA (=Orbicdloidea, Davidson ; see pp. 120, et seq.), we find a gene- 

 ral correspondence in the character of these impressions, although our knowledge 

 of the muscular scars of the brachial valve in that group is limited to a single 

 species, S. conica, Dwight, and in this case only to the two large central scars, 

 parted by a thickened median area and septum. In the absence of satisfactory 

 evidence concerning these important characters in the brachial valve of Orbicu- 

 LOIDEA, D'Orbigny, we may assume that their arrangement is indicated by what 

 is known in the case of Schizotreta, a group very closely allied in other 

 respects. 



In the pedicle-valve of Trematis there is an apparent correspondence in the 

 character of the pedicle-opening with the forms which have been referred 

 above to Orbiculoidea, D'Orbigny. In the discussion of these fossils it has 

 been shown that the aperture is not an oval perforation as in Discinisca, or a 

 fissure extenduig to the margin as in Trematis, but a tubular oblique passage, 

 most closely allied to that in Siphonotreta. The homology in this respect 

 then becomes remote, though distinctly traceable through the aberrant Discina 

 {Q^hlertella) pleurites, and perhaps is even more direct in the case of Linustrcem- 

 ella. Schizobolus, Discinolepis, ScHizocliANiA and perhaps Kutorgina conform 

 with Trematis in having the aperture a radial incision, and as the nature of the 

 pedicle-opening must be considered as of radical importance in determining the 

 taxonomy of the inarticulate genera, the groups named will fall into close 

 contiguity. It has been shown how closely Schizobolus is allied to Oholella 

 in' the character of its muscular anatomy, and that Kutorgina probably repre- 

 sents an incipient stage in the development of the Siphonotretoid pedicle-tube. 



