170 PAL-EONTOLOGY OF NEW- YORK. 



side extending downwards below the muscular cavity, and from Avhich 

 diverge the vascular impressions. 



The casts of the two valves preserve all the characters enumerated, and in a few 

 instances the vascular impsessions are well preserved. The strength of the parts 

 described is subject to some variation in different individuals, and from different 

 conditions of preservation. 



The smaller individuals bear much resemblance to 0. orbicularis^ as figured by 

 Barrande ( Silurische Brachiopoden aus Boehmen, ut citata, Tab. xx, f . 6 ) ; but 

 the beak is more incurved, and the muscular impression differs very essentially from 

 the one given by Barrande. 



Orthis tetragona of de Verneuil, as figured in Dunker & von Meyer's Palseonto- 

 graphica, pi. 37, f. 8, closely resembles some of the intermediate forms of this 

 species; but the ventral valve is not sufficiently elevated, and the striae appear to 

 be finer than these. 



The smaller individuals of this species have been mistaken for 0. perelegans, to 

 which it is closely related; but full-grown specimens have the dorsal valve more 

 distinctly sinuate and much less convex, while the central angular elevation of the 

 ventral valve is equally a distinctive character. Both these species have been referred 

 indiscriminately to O. elegantula, from which they differ in many important charac- 

 ters. Among the Lower Silurian forms, this has its representative in 0. testudinaria, 

 but is a larger species, the striae are finer and less distinctly fasciculate, and the 

 internal difierences are more striking than the exterior. 



Fig. 7, 8 a, b, c. Views of young individuals. 



Fig. 9, 10. Views of individuals which have a subquadrangular outline, with the dorsal 



valve deeply depressed in the centre and much elevated on each side, and the 



front margin strongly sinuate. 



( These forms will perhaps prove specifically distinct from the following ; but 



at present I shall consider them only as a variety, O. quadrans.) 

 Fig. 11, 12. Interior of the valves of the preceding variety. 

 Fig. 13, 14, 15. Individuals showing gradation in size, and some slight modification in 



proportions. 

 Fig. 16, 17. Full-grown individuals of this species. 

 Fig. 18. Interior of the ventral valve. 



Fig. 19 a, b. Interior of the dorsal valve, showing the muscular and vascular impressions. 

 Fig. 20, 21 a, 6, c. Ventral and dorsal yiews of casts, the vascular impressions, etc. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains; Catskill, Schoharie. 



