210 PALEONTOLOGY OF NEW YORK. 



process is erect, strongly arched on its anterior face, often very thick and 

 greatly elevated ; the edge of its posterior face is multilobate, the posterior 

 surface itself having a trilobed appearance. The muscular area is quadruplicate, 

 comparatively small and usually indistinct. A broad, low, median ridge extends 

 forward from the base of the cardinal process. The shell-structure is coarsely 

 fibrous and very strongly punctate, the perforations being large and generally 

 more abundant along the furrows between the striae. 



Dr. QEhlert's term Rhipidomys^Rhipidomella, which was founded on the Tere- 

 bratula Michelini, Leveille,* must be extended to include what proves to be the 

 largest of the subordinate divisions of Oethis. Shells of this type, though 

 attaining their greatest numerical and specific development in the Devonian 

 and Carboniferous, appeared in America as early as the age of the Clinton 

 fauna, in the typically developed species 0. circulus. In the Niagara group its 

 representation is limited to the species O. hybrida, a shell which rarely, if ever, 

 attains the size in America that it does in the Wenlock of Great Britain and 

 the Island of Gotland. From this point onward the species in the following 

 faunas rapidly multiply ; the Lower Helderberg fauna containing the forms 0. 

 oblata, 0. discus, 0. eminens, 0. tuhulostriata, and that of Oriskany, 0. musculosa, 

 the largest member of the group, with an extravagant development of the 

 muscular scars. They reach their culmination in the Devonian, gradually 

 declining and finally disappearing with the close of the Carboniferous age. In 

 the faunas of the latter there is occasionally manifested a variation in the ex- 

 pression of these species, without change in the essential points of structure. 

 The 0. Penniana, Derby, from the Coal Measures of Itaituba, Brazil, has an 

 elongate-subovate outline, and a very short hinge-line ; Orthis Pecosi, Marcou, 

 is a shell of much the same character, while in 0. incisiva, Waagen, and 

 0. dnbia, Hall, the hinge-line has become so short that the shell is actually tere- 



* Waagen had" previously regarded this species as the type-form of a well defined group in the Car- 

 boniferous, to which he referred the three species occurring in the Salt-Range : 0. ccyi-allina, Waagen, 

 0. Pecosi, Marcou, 0. incisiva, Waagen. (See Memoirs Geolog. Surv. India; PaliEontologia Indica, 

 Ser. xiii, vol. iv (fas. 3), p. 5(32. 18S4.) 



The tei'ui Rhipidomys having already been used for a genus of mammals, Dr. ffiHLERT has proposed the 

 term Rhipidomella. 



