BRACHWPODA. 
227 
SUPPLEMENTARY NOTE ON THE G-ENUS ORTHIS. 
In the foregoing list of species, the form currently known in this country as Orthis flabel- 
lulum, from the Clinton-Niagara fauna, is arranged with the typical division of the genus. 
In a previous provisional arrangement it was placed under the group Dinorthis, in associa¬ 
tion with 0. pectinella. There are excellent reasons for its position in either association. With 
the typical orthids it agrees in the simple plications of the surface, and in the character of 
the muscular impressions in both valves. Attention is especially directed to this conformity 
in the pedicle-valve, where the outline of the muscular area is the more variable throughout 
the entire genus; and the figure 40, upon Plate V,* shows its oval form and restricted ex¬ 
tent, similar to that seen in Orthis calligramma , var. Davidsoni (fig. 5), and Orthis costalis (fig. 
15.) The species is however a distinctly resupinate shell, as shown on Plate V, fig. 39; the 
pedicle-valve, in the umbonal region, is quite as deep as the brachial, but is gently depressed 
over the pallial region, while the brachial valve remains convex throughout. To associate 
this species with the typical forms of Orthis, would open that division to the reception of 
resupinate shells, thus destroying its homogeneity. On the basis of external characters the 
shell belongs to the group Dinorthis, but it does not agree with Orthis pectinella in its mus¬ 
cular impressions, so that it must be regarded as a form connecting the typical Orthides 
with Dinorthis ; though, in geological time, appearing at the end of the two groups. 
The original identification of this Niagara species as Orthis flabellulum, (a) ? was made 
in 1843, f from comparison with the figure given by Sowerby, in Murchison’s Silurian Sys¬ 
tem (pi: xxi, fig. 8). This figure did not indicate with clearness either the resupination of 
the shell or the outline of the muscular scars, and the identification, made with doubt, has 
been accepted as final. 
The elaborate illustration of the British species of this name given by Mr. Davidson at a 
later date,J shows that it is strongly resupinate and has a subquadrate muscular area in the 
pedicle-valve, features which at once associate it with Dinorthis, and it is further evident 
that it is a close ally in all its characters with the American species 0. pectinella , though in 
the latter the bifurcation of the plications, which appears to be normal for O. flabellulum , 
Sowerby (var. (3 ), is of less frequent occurrence. These two forms are from equivalent 
horizons. For nearly fifty years an erroneous identification, though made with the best 
lights of that time, and expressed with doubt, has been current in American literature. 
In the Twentieth Report on the New York State Cabinet of Natural History (1867, p. 397), 
the term Orthis flabellites, Hall, was used for the species in a list of the fossils occurring in the 
limestones of the Niagara group in Wisconsin, Illinois and Iowa; and although no discussion 
or explanation of the reference was there given, the name was intended to supercede the use 
of 0. flabellulum. 
* An additional figure of tills valve, showing these characters with greater distinctness, will be given upon a supple¬ 
mentary plate at the close of this work. 
f Geology of New York; Report Fourth District, p. 107. 
;f 1859. Silurian Brachiopoda, pi. xxxiv, figs. l-12a. 
