ANIMALS. 107 
exclusively to the Silurian period. The type of the genus is peculiar to the Silurian 
rocks of Gothland; while Dicelosia sinuata (Spirifer id.), G. B. Sow., and D. varica 
(Delthyris id.), Conrad, are respectively characteristic of the equivalent deposits of 
Dudley and North America. 
Passing over the genus Chonetes, originally established by Fischer de Waldheim 
(but subsequently elucidated by Dr. de Koninck) on the Terebratulites sarcinulatus of 
Schlotheim, and which is principally distinguished from Strophomena by having a row 
of spines running along the angle of the area, and an open fissure, I propose in the 
next place to notice another generic group of more immediate interest in connexion 
with the present Monograph. 
Genus Streptorhynchus, King. 
Diagnosis.—A Strophomenidia; ineequivalved ; striated or ribbed; with the hinge 
approximating or equal to the width of the valves. Umbones more or less divaricating ; 
the large one irregularly twisted. /sswre covered with a deltidium. Dental plates 
small; projecting more at the base of the area than at its apex. 
Type, Zerebratulites pelargonatus, Schlotheim. 
This genus, which consists of a small number of known species, is allied to 
Orthis and Orthisina; but it differs from both in the peculiar twisted character 
of its umbone, and the smallness of its dental plates. As regards the last character, 
the dental plates in the genera just noticed are the very reverse of those prevailing in 
the genus under consideration; as in Orthis (and it is the same with most of the 
Palliobranchiate genera furnished with these structures) they project more at the apex 
of the area (occasionally extending considerably into the umbonal cavity) than at its 
base, that is, immediately adjoining the teeth,—which is evidently consequent on the 
former portion gradually increasing, in a forward direction, as the shell advances in 
age; whereas in Streptorhynchus they are more projecting at the base of the area than 
at its apex, where they still retain their original size,—proving that they have no 
growth independent of the increment of the teeth. 
Comparing Orthis arachnoidea, and O. crenistria, Phillips, with Streptorhynchus senilis 
(Spirifera ad.), Phill., this genus would seem to be aberrant from the normal Orthises. 
Streptorhynchus senilis, Phill., 8. pelargonatus, Schl., and S. spurius (Spirifer id.), 
Braun., are the only forms with which I am acquainted; and they respectively 
characterise distinct, yet consecutive formations,—the Carboniferous, Permian, and 
Triassic. 
1 From stpemrw, I bend or twist, and puyyos, a beak. 
2 Vide De Verneuil’s figures of Orthis eximia and O. Olivieriana in Geol. Russ., vol. ii, pl. xi, 
figs. 2a and 36. 
