84 MEMOIRS OF THE STATE MUSEUM 



The first two stages of the shell figured by Mr. BROOKS* represent 

 free animals, and measure .24 and .3 mm. in length, respectively. The 

 shell became attached by the pedicle only upon attaining a length of 

 2.5 mm. 



Most of the fossil forms have furnished evidence, either from 

 actual elemental specimens or from the apical portions of subsequent 

 incipient stages, that the true initial shell did not reach a size of more 

 than .5 mm. in length. Soon after this period, the characters of each species 

 become developed and impressed upon the shell more or less gradually. 



Even such distinct groups as Rhynckonella, Spirifer, Athyris, 

 Rhynchotreta, Anastrophia, Nudeospira, and the Meristoids, in their initial 

 stages, approach one another so closely that they can be determined 

 only from comparatively trivial features. They are alike in form, con- 

 tour, convexity, beaks, and cardinal area, and the only marked differences 

 are to be found in the faint indications of plications, striae, folds, and 

 sinuses. For species of some genera, as Orthis, Whitficldia, and Spirifer, 

 even these characters are not determinative, and it is impossible to refer 

 certain embryos to their proper place. 



From the foregoing statements, it would naturally be inferred, that 

 the species which, at maturity, present characters 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 reversed species above examined, belong- 

 ing to the genera Anastrophia, Strophonclla, and Streptis. 



Beginning with the initial shell having a circular outline and de- 

 pressed valves, we find 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 



* Johns Hopkins University ; Chesapeake Zoological Laboratory. The Development of 

 Lingula and the Systematic Position of the Brachiopoda, plates i and 2. 1879. 



