BRACHIOPODA. 227 



SUPPLEMENTARY NOTE ON THE GENUS ORTHIS. 



In the foregoing list of species, the form currently known in this country as Orthis flahel- 

 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 

 iu 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 oalligramma, 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, t from comparison with the figure given by Sowerbt, 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 0. flabellulum, 

 Sowerby (var. fi), 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 flabdlites, 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 O. flahellulum. 



* Ad adJitioDal figure of tliis valye, showing these character with greater dfistinotness, will be given upon a supple- 

 mentary plate at the close of this work. 



t Geology of New York; Report Fourth District, p. 107. 

 X 1859. Silurian Brachiopoda, pi. xxxiv, tigs. l-12a. 



