58 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



there is a triangular slit below, whicli may sometimes be filled by deltoidal 

 pieces. The dorsal valve has a broad flattened cardinal process, which dis- 

 tinguishes it from those just mentioned, or from R.ferita and others referred 

 to Retzia. These features have been illustrated in the Twelfth Report on 

 the State Cabinet, and in the third volume of the Palaeontology of New- 

 York. 



The following illustrations are repeated here, for comparison with those 

 already given. 



Fig. 12. 



FiG.U. 



Fig. 12. Dorsal view of a specimen of Rhyr^chospira formosa, natural size. 



Fig. 13. Interior of dorsal valve enlarged two diameters, to show the broad cardinal process j 

 which covers the extremity of the beak, the bases of the crura c, and the short 

 medio-longitudinal septum. 



Fig. 14. Profile view of same, showing the cardinal and crural processes. 



Fig. 15. 



Fig. 16. 



i?. ./ 



Fig. 15. Interior of ventral valve, natural size; shoeing the teeth, the deltidium, and the 



foramen. 

 Fig. 16. The upper part of the two valves, connected in their natural relations to each other, 



the ventral valve below ; showing the teeth and sockets, the bases of the crura, and 



septum of the dorsal valve. 

 Fig. 17. Longitudinal section, showing the foramen and deltidium; the cardinal process of the 



dorsal valve lying beneath the latter. The crura are first bent downwards, and then 



recurved into the dorsal valve, continuing in the spire. The descending process *c is 



united with a similar one from the opposite side, which together form a connecting 



band between the two spires. 



In the present state of our knowledge of the structure of these species, 

 I can see nothing to be gained to science by uniting all these diff"erent forms 

 under a single generic term. 



It is clear that the Genus Retzia must be restricted to one of three very 

 distinct groups of species here presented ; and should it be found to em- 

 brace, in the range of its typical species, some of those I have described 

 as Trematospira, it cannot, by any proper extension, include those of 



