392 STUDIES IN EVOLUTION 



there are some points of difference. The investigations 

 referred to were made upon one (or more) Mesozoic species of 

 Terebratula, Rhynchonella, and Spiriferina, specific designa- 

 tions not given. The illustrations of Terebratula (figures 

 1 a, b, c, and column A, a, /3, 7, S) show in effect the char- 

 acters seen in Whitfieldella nitida, Meristina Maria, and 

 others ; those of Rhynchomlla, early stages of similar charac- 

 ter, resulting in a deltidium amplectens, such, as just observed, 

 have not been found in Paleozoic Rhynchonellse. 



In Spiriferina, according to M. Deslongchamps, the 

 pseudo-deltidium is produced by the gradual development 

 of a single plate in the apex of the triangular opening, in- 

 creasing downward with age, a very distinct mode of for- 

 mation from all the others, and open to verification in the 

 species described by that author, as his figures make no 

 allowance for a pedicle-sinus or perforation, a feature, though 

 not of frequent occurrence in the Spiriferidse, yet one neces- 

 sary to account for. 



The writers have examined specimens of Spiriferina pinguis 

 Deslongchamps, S. rostrata Schlotheim, and S. Walcotti Sow- 

 erby, and find that these species, at least, develop triangular 

 deltidial plates. Those in Spiriferina pinguis and S. Walcotti 

 are comparable with the same parts in Spirifer perlamellosus 

 and cumberlandice, and their form and mode of growth are 

 expressed by the outlines 2", 3", on page 384, and is further 

 shown in figure 130. Additional growth causes the plates to 

 unite along the median line, obliterating the partially formed 

 pedicle-perforation, and subsequent increment can naturally 

 take place only along their lower free edges. 



The remarks on Spirifer radiatus and S. crispus indicate 

 that the development of the plates in this member of the same 

 family is quite in harmony with the process as seen in the 

 rostrate forms generally. 



The following is the summarization of Deslongchamps's 

 conclusions, as given by himself: 



(1) The deltidium is one of the most important features 

 in the articulated brachiopods. 



