62 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



lustrated the characters of the genus,* and we are now enabled 

 to make satisfactory comparisons with some of the American 

 species. 



Most of the shells of this genus are closely related to Strophomena : 

 others have the external characters of Orthis, and are separable from that 

 genus principally by the closed fissure ; but the strias do not usually 

 curve up to the hinge-line, nor are th^ surfaces poriferous or punctate, in 

 the species which I have examined. Like many of the palaeozoic genera, 

 they undergo considerable changes in the lapse of time, and the Permian 

 0. pelargoiiatiis has little similarity with some of the more ancient forms, 

 which appear referable to this genus. 



The shells of this genus are semicircular or semielliptical, concavo-convex 

 or plano-convex, and sometimes with both valves convex. They are external- 

 ly striated with rounded bifurcating threadlike striae, which are crossed by 

 fine concentric lines ; and in some species the stronger striae are distant, 

 with finer radiating and concentric striae cancellating the intermediate spaces. 

 The ventral beak is sometimes prolonged and bent or twisted, and the 

 fissure is closed or partially closed by a deltidium. A narrow area often 

 exists on the dorsal valve, but this is not a constant character. 



In this description, I have reference to the species before me, the half of 

 which have not the beak prolonged or distorted. The exterior striation, in 

 the older and the younger forms,! presents considerable variation ; which 

 may be characteristic, or this gradation may be apparent only in the series 

 of specimens which I possess. 



Referring to the structure exhibited in Mr. Davidson's illustrations, I 

 can have no difiiculty in recognizing such forms as Strophomena ivoohvor- 

 thana among the true Streptorhynchi ; while the S. pandora (Billings) 

 of the Schoharie grit and Upper Helderberg limestone, is an allied species. 

 Tlie Streptorhynchus arctostriati and S. alternat^ of the Hamilton groupt 

 approach more nearly to the Carboniferous varieties of European and 

 American species. 



In the S. ivoolworthana, the dental plates of the ventral valve are large 

 and strong, widely diverging, gradually merging into the thickened margin 

 of the flabelliform muscular impression. The muscular impression presents 

 no important differences from that of many species of Orthis, or from that 

 of Strophomena alternata. The foramen is partially closed, and the cardinal 

 process of the opposite valve is seen filling the space below the deltidium. 

 The dorsal valve is convex, while the ventral is flat or concave. The cardi- 



* Monograph of British Permian Brachiopoda, 1857, p. 29, and Plate ii, f. 32 - 42. 

 Also British Carboniferous Brachiopoda, 1860 - 61, Plates 25, 26 &c 27. 



t Those from the older and the younger formations. 



X Originally described as Orthisina in the Thirteenth Report on the State Cabinet, 

 pp. 80 & 81. The extreme variation to which the known European species are subject, 

 as shown by Mr. Davidson, may lead us to suspect the unity of these two, though 

 possessing well-marked differences in all the individuals examined. 



