BRACHIOPODA. 323 



a good excuse for associating them closely, as has been done by (Eiilert, who 

 places the latter genus among the Inarticulates Eichwaldia presents a pecu- 

 liar modification of the pedicle-passage, and all its essential characters, acquired 

 at an early Silurian age, were maintained to the close of the Upper Silurian 

 without substantial variation. The origin of Eichwaluia is, at present, but a 

 matter of conjecture; such resemblance as it bears to Trimerella, in its incip- 

 ient articulating apparatus, seems to be only an instance of isomorphy. 



The second main division of the Inarticulate genera is composed of those in 

 which the pedicle-aperture, in the immature stages or in primitive adult condi- 

 tions, takes the form of a marginal incision of the pedicle-valve, but becomes 

 enclosed in the shell-substance in later stages of growth. To this group Waaoen 

 applied the term Diacaulia* (or Discinacea, 1883), which, like Mesocaulia, is an 

 admirable expression of the significance of the pedicle-passage. The name 

 Neotremata was subsequently introduced by Beecher (1891) as an ordinal term 

 for not only such forms as these, but also for those like Crania, of whoso fixation 

 by means of a pedicle there is yet no evidence. 



The mode of development and enclosure of the marginal incision in the genus 

 Orbiculoidea has already been demonstrated,! and it has been shown that 

 ffiHLERTELLA, Trematis and ScHizocRANiA, which have an unenclosed aperture 

 at maturity, are primitive conditions through which Orbiculoidea passes in the 

 development of the individual. These primitive adult conditions occur in 

 various faunas from the primordial (Discinolepis) to the Lower Carboniferous 

 (ffinLERTELLA), and wliilc these genera might be conveniently as.sociated on the 

 basis of this feature, it is doubtful whether such grouping would be a natural 

 one, or a proper expression of the relations of these forms to the various con- 

 temporary mature types. 



* This name was originally jirinted Daikadlia, probably a typographical error in the spelling' of the 



first syllables. 



Waagen, following usage in the employment of the terms Ltopomata and Arthhoposiata as ordinal 

 designation.^, subordinate only to the name of the Class, Brachiopoda, iuti-oduced Mksocaulia and Diacai-lia 

 as names of sjiborders. It is a purely arbitrary matter whethei- the former terms be regarded as designa- 

 tions of orders or subclasses. They are, in either case, inferior in the first degree to the Class itself. Hence 

 Ihe fact that Waagex employed the lattei- terms as suborders is no ground for rejecting either of them for a 

 later name having the same significance. 



t Volume VIII, Part I, loc. cit. 



