DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 391 



incidentally, by various authors. The present results, though 

 derived from the species of a single fauna, must not be given 

 too limited an application, for they involve nearly every 

 important family of Paleozoic articulate brachiopods, and 

 it may be tentatively assumed that, as a rule, the essential 

 features of variation observed in any member of a genus 

 will hold good of the other members. In regard to the 

 develoj^ment of the characters of the pedicle-passage, i.e., 

 the deltidial plates and the foramen, there is good reason 

 to regard the process as substantially identical in all the 

 genera represented, making the necessary allowance for 

 the peculiar variation seen in the Strophomenidce, which 

 may not, however, prove it an exception to the general 

 statement.* 



The various terms which have been sometimes applied to 

 the condition of the deltidial plates in the rostrate genera — as 

 deltidium amplectens, when the foramen is entirely surrounded 

 by the plates, as in various Mesozoic RhynchonellEe (but in 

 no Paleozoic species, as far as known); deltidium sedans, 

 when the plates bound the foramen only on the lower 

 side, the upper side encroaching on the substance of the 

 umbo, as in Terehratula Whitfieldella, etc. ; deltidium dis- 

 cretum, when the plates do not come into contact, as in 

 Terehratella, some species of Rhynchonella, etc., — must be 

 regarded as having no further significance than to express 

 the existing condition of the foramen and deltidial plates 

 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 those 

 here expressed (Note sur le d^veloppement du deltidium chez 

 les brachiopodes articul^s : Bull. Soc. Geol. France, 2" ser. t. 

 xix, pp. 409-413, pi. ix, 1862), but with his conclusions 



* See footnote, p. 393. 



