ORTHIDES OF THE PORTAGE AND CHEMUNG GROUPS. 
61 
This species has many characters in common with Orthis tulliensis; differing 
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, maybe only differences caused 
by the conditions of life in the 0 . tulliensis at a later geological period. 
The Orthis tulliensis occurs in the Tully 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 Tully limestone it is associated chiefly 
with Rhynchonella suicuboides , a fossil restricted, as far as known, to that horizon. 
Between the Tully limestone and the Chemung group we find interposed the 
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 axi§: while the 
dorsal area is not so high, and the sinus in front is more abrupt. These features 
are shown in figs. 2 a - 2 i 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 l, o, p (loc . tit.), with fig. 5 of Plate 
vii, Yol. 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 characteristics are reliable 5 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. 
