DEVELOPMENT OF SOME SILURIAN BRACIIIOPODA 393 



( 2 ) As far as Jurassic species are concerned, the deltidium 

 may suffice to characterize the families. 



(3) In the various stages of development of this part the 

 aspect of the shell is entirely changed. 



(4) The deltidium appears under three important modifi- 

 cations: (A) Development below the peduncular arm, char- 

 acterizing the Terebratulidae ; (B) development above the 

 peduncle, Spiriferidse ; (C) mixed development, surrounding 

 the peduncle, Rhynchonellidcie. 



(5) The stage at which the development is arrested or the 

 exuberance of development 



may suffice to characterize ^•'^0 



sections under the families. 



It has just been shown 

 that conclusions 2, 4, and 

 5 are not capable of the 

 extended application which 

 Deslongchamps has given 

 them. 



A preceding remark, 

 that the course of develop- 

 ment of the deltidial charac- 

 ters throughout the genera 

 here discussed may be considered as fundamentally uniform, 

 calls for explanation in its application to the Orthidoe and the 

 Strophomenidoe.* In the latter forms it has been shown that 

 the remarkable development of the pedicle-sheath is primary, 

 and is invariably more or less atrophied with age, and probably 

 functionally inactive at maturity. Hence the retention of 

 this sheath in an}^ species at maturity is the perdurance of 

 what must serve as an embryonic character within the limits 

 of this family. It cannot escape observation that the pedicle- 

 sheath is in analogy with the entire rostrate umbo of the 



[* The following correlations {aupra) of the characters of the canliual area 

 were made before the true significance of the pedicle-sheath in tlie stropliomenoid 

 genera was understood. The subject is fully discussed (ante) in the second part 

 of the Development of the Brachiopoda.] 



Figure 130. — Deltidial development in 

 1, 2, Spirtferina pinyuis Deslongchamps ; 

 3, 4, Spiriferina Wnlcnttl Sowerby ; 

 5, Spiriferina rostrata Schlotheira. 



