ORTHIDES OF THE PORTAGE AND CHEMUNG GROUPS. 61 



This species has many characters in common with Orthis tulliensis; diifering 

 chiefly in the broad flattened or depressed ventral valve, which has a broader and 

 more defined sinus along the centre, and also a large and deeply divided muscular 

 impression which is sometimes lobed. It is possible that these characters, which 

 are subject to variation in the specimens before me, may be only differences caused 

 by the conditions of life in the 0. tulliensis at a later geological period. 



The Orthis tulliensis occurs in the TuUy limestone in the central part of the 

 State, principally in Onondaga and Cayuga counties. It has not been found in the 

 Hamilton group ; and in the thin bands of TuUy limestone it is associated chiefly 

 with Rhynchonella subcuboides, a fossil restricted, as far as known, to that horizon. 



Between the Tully limestone and the Chemung group Ave find interposed th^ 

 Genesee slate and Portage group, which together make a thickness of eight 

 hundred to one thousand feet : in these beds, no species of Orthis is at present 

 known. 



The species of this type, from the 0. multistriata of the Lower Helderberg 

 group to this one from the Chemung group, bear many characters in common, and 

 under some circumstances it might be difficult to distinguish them. The 0. multi- 

 striata has subequal striae, approaching in this character to the 0. tulliensis, which 

 it likewise resembles in form ; but the beak of the ventral valve is more produced, 

 and the area is less divergent from the plane of the longitudinal axis ; while the 

 dorsal area is not so high, and the sinus in front is more abrupt. These features 

 are shoAvn in figs. 2 a - 2 t of Plate xv, Vol. iii, Pal. N.York. In the same species, 

 figs. 2 k - 2 r, the muscular areas are similar to those of 0. tulliensis; that of the 

 ventral valve being a little more strongly lobed and more angular in outline, while 

 that of the dorsal valve is but slightly different in form. The vascular impressions 

 proceeding from the base of the muscular area are, however, quite distinctive in all 

 the casts seen, and may be compared in figs. 2 I, o, p ( loc. dt.), with fig. 5 of Plate 

 VII, Vol. iv, which represents the constant character of 0. tulliensis as well as of 

 the western form, 0. iowensis. The differences between the 0. tulliensis and 0. 

 propinqua have already been pointed out, and, in a considerable number of indivi- 

 duals, these characteristips are reliable ; but when we find crushed and distorted 

 specimens of the two species, they are not easily distinguished. 



The Chemung specimens are never entire : they are generally distorted, and the 

 shell is rarely preserved to any extent. In the distorted specimens, the muscular 

 impressions of course participate in the abnormal appearance ; but in several well 

 preserved" specimens of the ventral valve, there is a much greater variety of form 

 and proportions of this part than has been observed in any one of the species 

 before described. 



