ANIMALS. 107 



exclusively to the Silurian period. The type of the genus is peculiar to the Silurian 

 rocks of Gothland ; while Dicalosia simiata {S])irifer id.), G. B. Sow., and B. varica 

 {Belthyris 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 Streptorliynclius} King. 



Diagnosis. — A Strophomenidia ; insequivalved ; striated or ribbed ; with the hinge 

 approximating or equal to the width of the valves. Vmhone's more or less divaricating ; 

 the large one irregularly twisted. Fissure covered with a deltidium. Dental plates 

 small : projecting more at the base of the area than at its apex. 



Type, Terebratulites 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 Streptorhpichus 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 0. crenistria, Phillips, with Streptorhynchus senilis 

 {Spirifera id), Phill.. this genus would seem to be aberrant from the normal Orthises. 



Streptorhynchus senilis, Phill., S. pelo.ryonatus, 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 sTpeTTTU), I bend or twist, and pyy^os, a beak. 



2 Vide De Verneuil's figures of Orthis eximia and O. Olivieriana in Geol. Russ., vol. ii, pi. xi, 

 figs. 2 a and 3 b. 



