878 PALEONTOLOGY OF NEW-YORK. 



The following figures ai-o ilUistFations of one of the typical species of Riitn- 

 CHOsriRA, from the Lower Helderberg group. 



lUlrNCBOSPIRA FOBMOSA. 

 I 



5 6 



Fia. 1. Dorsal view, natural size. Fia. 2. Interior ot a dorsal valve, showing the cardinal process, etc. 

 Fig. S. Profile of figure 2, showing the recurved cardinal process, 

 Fio. .<. The ventral vnlve of the same species. 



Fio. 5. A longitudinal section, showing the articulation of the valves, the origin of one of the crura, 

 the spire, etc. 



Fia. 6. The upper part of two valves, showing the mode of articulation. 



GeNLS IVlCLEOSPIRA* (Hall). 



This genus was proposed by me to include a few small species of fos- 

 sils which had heretofore been referred to Spirifer, Atrvpa, Oktuis, etc., 

 but which do not possess the true characters of either of these genera. 

 Since we know the peculiarities of its internal structure and the accom- 

 panying exterior, together with the punctate texture, there is no longer 

 any difficulty in recognising these shells among the other Brachiopoda. 



The following are the important characters of the genus, as determined 

 from the examination of the species of the Niagara, Lower Helderberg 

 and Hamilton groups. 



Shell depressed spheroidal or transversely elliptical, more or less 

 gibbous or ventricose, furnished with internal spires as in Spirifera ; 

 hinge-line shorter than the width of the shell; cardinal extremities 

 rounded : valves subequal, articulating by teeth and sockets. Ventral 

 valve having the beak slightly extended beyond the opposite valve, and 

 beneath it a triangular depression or area, which has sometimes a 

 shallow semispoonshaped form ; on each side of which, at the base, is a 

 strong tooth. A narrow ridge or septum along the centre of the inner 



• This genus was first described in the third volume of the Paleontology of New- York, p. 219, and 

 is copied on pages 24 and 25 of the Twelfth Report on the State Cabinet. 



