17 © 
PALAEONTOLOGY OF NEW-YORK. 
side extending downwards below the muscular cavity, and from which 
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 strise 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 0 . elegantula , from which they differ in many important charac¬ 
ters. Among the Lower Silurian forms, this has its representative in O. testudinaria , 
but is a larger species, the strise are finer and less distinctly fasciculate, and the 
internal differences 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, b, c. Ventral and dorsal views of casts, the vascular impressions, etc. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains; Catskill, Schoharie. 
