LOWER HELDERBERG ROCKS. 331 



i 



perforate upon the under side. Internal spires, as in Spirifer, showing ten or twelve 



turns on each side. 



These shells are rarely or never found in separated valves, like most of the other 

 Brachiopoda ; and the cause of this is clearly discernible, when we know the 

 character of the hinge in which the recurved cardinal process extends into the 

 curved beak of the opposite valve, while its narrow base is closely clasped by the 

 curved teeth of the ventral valve, forbidding any dislocation of the two valves 

 without a fracture of the parts. The extent to which this cardinal process penetrates 

 beneath the beak of the ventral valve may be seen by carefully breaking away the 

 shell of that valve below the beak; which, if skilfully performed, will leave the 

 spoonshaped process projecting beyond the shell. 



This species resembles JVucleospira pisiformis of the Niagara group; but the form 

 is slightly more extended, the beak of the ventral valve more elevated and incurved, 

 and the false area more distinct. It varies in size from the dimensions of fig. 1 a to 

 A ; usually presenting a very rotimd form, but not unfrequently compressed as in 

 fig. 1 g and h, and sometimes extremely ventricose as in 1 i, k. 



Fig. 1 a, b, c. Individuals of Bmall size. 

 Fig. 1 d, e. Casts of larger forms. 

 Fig. If, g, h. Specimens of large size. 



[ Tlie figures 1 / - o do not belong to this species.] 



PLATE XXVIII B. 



Fig. 2 a, b, c. Illustrations of a form having the beak more elevated than usual. 



Fig. 3 a, b. The ordinary form of this species. 



Fig. 3 c, d. An individual preserving the remains of the fine hair-like spines which cover 

 the surface of perfect specimens. 



Fig. A a, b. A cast of the interior. 



Fig. 5. The interior of a ventral valve preserving the internal spires. 



Fig. 6 a. The dorsal valve, showing the extended cardinal process and the longitudinal 



septum. 

 Fig. 6 6. Profile of the same. 

 Fig. 6 c, d. Enlargement of the preceding figures. 

 Fig. 7 a, 6. Interior of the ventral valve, and profile view of the same. 

 Fig. 8. Interior, showing the articulation of the valves ; t, t, the cardinal teeth ; J, the 



cardinal process ; B, B, the brachial processes. 



Fig. 9. Diagram showing a longitudinal section of the valves, the articulating teeth, the 

 cardinal process, spire, and longitudinal septa s, s. 



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

 group : Helderberg mountains, Schoharie, Cherryvalley,inNew-York; Cumberland 

 in Maryland, etc. 



