DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 387 



this period the characters of each species become developed 

 and impressed upon the shell more or less gradually. 



Even such distinct groups as Camarot&chia, Spirifer, Athy- 

 ris, Rhynchotreta, Anastrophia, Nudeospira, and the meris- 

 toids, in their initial stages, approach one another so closely 

 that they can be determined only from comparatively trivial 

 features. They are alike in form, contour, convexity, beaks, 

 and cardinal area, and the only marked differences are to be 

 found in the faint indications of plications, strias, folds, and 

 sinuses. For species of some genera, as Dalmanella, Rhipid- 

 omella, Meristina, and Spirifer, even these characters are 

 not determinative, and it is impossible to refer certain 

 embryos to their proper places. 



From the foregoing statements it would be naturally 

 inferred that the species which at maturity present char- 

 acters abnormal to the typical structure, have been diverted 

 from the harmony which existed in the incipient stages, with 

 the other members of the group. This has been shown to be 

 the case in all the foregoing reversed species examined, belong- 

 ing to the genera Anastrophia, Strophonella, and Mimulus. 



Beginning with the initial shell having a circular outline 

 and depressed valves, it is found that subsequent growth takes 

 place about the periphery, and in the majority of species the 

 convexity is gradually increased until maturity is reached. 

 This assertion does not hold true for such forms as the 

 StrophomenidaB, which vary in convexity, either very slowly 

 or not at all, up to individuals about half -grown, when the 

 valves become more or less deflected and often concave. 

 Such reversion in the shell is in conformity with the de- 

 generacy which is traced in the development of the cardinal 

 area and pedicle-sheath, mentioned on a subsequent page. 



The observations of Brooks and Morse, in the works cited, 

 show that in both the hingeless and the hinged brachiopods, 

 as represented by Linyula and Terebratulina, the early 

 stages of the shell approach a sub-circular outline, and Brooks 

 remarks (op. cit. p. 43), that " the recent and fossil shells of 

 the various species of Crania, Lingula, Lingulella, and Obolus, 



