SILURIAN BRACHIOPODA 89 



side encroaching on the substance of the umbo, as in l^erebratula, Meris- 

 tina, etc., dcltidinm (Uscretum, when the plates do not come into contact, as 

 in Tcrcbratella, some species of RJiyiichonella, etc., must be regarded as 

 having no further significance than to express the existing condition of the 

 foramen and deltidium in any given specimen, that is, as indicating a stage 

 of development, not necessarily a generic or even specific character. 



The observations of M. EUGENE DESLONGCHAMPS upon these features 

 are of much value, and in most respects, as far as carried out upon related 

 forms, are in harmony with our own (Note sur le de'veloppement du deltid- 

 ium chez les brachiopodes articules : Bull. Soc. Ge'ol. France, 2 e Sen T. xix, 

 pp. 409-413, pi. ix, 1862), but with his conclusions, we are forced to 

 differ in some respects. The investigations referred to were made upon 

 one (or more) Mesozoic species of Tercbratula, Rhynchonella, and Spirifer- 

 iua, specific designations not given. The illustrations of Terebratu/a (figs. 

 id-, b, c, and column A, a, ft, y, tf) show in effect the characters seen in 

 Meristina nitida, Whitficldia maria, and others ; those of Rhynchonella 

 early stages of similar character, resulting in a deltidium amplectens, such, 

 as just observed, we have failed to discover in paleozoic Rhynchonellae. 



In Spiriferina, according to the author, the deltidium is produced by 

 the gradual development of a single plate in the apex of the triangular 

 opening, increasing downward with age, a very distinct mode of formation 

 from all the others, and open to verification in the species described by M. 

 DESLONGCHAMPS, as his figures make no allowance for a pedicle-sinus or 

 perforation, a feature, though not of frequent occurrence in the Spiriferidae, 

 yet one necessary to account for. 



We have examined specimens of Spiriferina pinguis, Deslongchamps, 

 S. rostrata, Schlotheim, and S. walcotti, Sowerby, and find that these 

 species, at least, develop triangular deltidial plates. Those in Spiriferina 

 pingnis and S. u<alcotti are comparable with the same parts in Spirifer 

 perlamellosus and S. cumber landice, and their form and mode of growth 



12 



