148 PERMIAN FOSSILS. 
length, and nearly five eighths in width: widest generally a little in front of its medio- 
transverse line: somewhat obtusely rounded in front, and tapering behind: rather 
compressed: posterior margins rounded: anterior margins sharp. Large valve 
depressed in its anterior half, and slightly rounded posteriorly; with a broad, some- 
what flattened, shallow simws in the medio-longitudinal region: wméone moderately 
prominent and incurved : foramen moderate in size ; truncating the point of the umbone ; 
and bounded inferiorly by the apex of the deltidium. Small valve obtusely ridged in 
its medio-longitudinal region ; with the lateral surfaces sloping rapidly to the margins. 
This is such a remarkably variable species, that it is difficult to draw up more than 
an approximate diagnosis of it; and for the same reason I have been compelled to 
represent more specimens than would have been necessary under ordinary circum- 
stances. Figures 30 to 43, inclusive, Plate VI, represent the principal varieties. 
Figs. 30, 32, and 37 (different specimens) are assumed to represent the typical or 
normal forms. 
Figs. 31, 39 (one specimen), and 35 (young), a narrow variety. 
Figs. 33, 36, and 43 (one specimen), a compressed wide variety, with a broad and 
very shallow median sinus. Sp., Zerebratulites latus, Schl. 
Fig. 42, a variety, with a very deep median sinus. 
Fig. 34, a variety, with a narrow umbonal region, and a rounded anterior half. 
Schlotheim considered some of the above varieties as species. I have no decided 
objection to this view; but I feel myself utterly unable to separate one form from 
another, they merge so imperceptibly into each other. It has therefore been deemed 
the best plan to unite them under only one of Schlotheim’s names,—the one generally 
received. 
Terebratula plica, Kutorga, found in the Mountain Limestone of Sterlitamak is 
considered by De Verneuil to be a variety of the present species: they undoubtedly 
resemble each other in some respects; but I have not yet seen any specimens of the 
latter with the lateral marginal line so strongly curved as in the specimen represented 
by Kutorga, in the ‘ Verh. der Min. gesellsch. St. Petersb.,’ 1842, pl. v, fig. lle. The 
specimen which I have partially represented in Pl. VI, fig. 42, offers an approach to 
it; and Dr. Geinitz has figured a specimen offering a still nearer approximation 
(Versteinerungen, pl. iv, fig. 27 ¢): so that I am somewhat inclined to De Verneuil’s 
opinion. If Fischer’s Zerebratula Qualeni be correctly represented by Kutorga in the 
work cited, I have considerable doubt of its being the same as Hpzthyris elongata, 
having never seen any specimens so narrow in the umbonal region: there is an 
approximation to it, however, in the specimen represented in Pl. VI, fig. 34. 
Some specimens appear to be identical with De Verneuil’s Zerebratula fusiformis, a 
Carboniferous species. Since my plates were engraved, I have procured specimens at 
Tunstall Hill apparently undistinguishable from the one represented in the ‘ Geology of 
Russia,’ vol. ii, pl. ix, fig. 8a, 6. Specimens from the Carboniferous Limestone of 
