DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 397 



shell, seem to warrant the assertion that the surface orna- 

 ments do not appear until the second or a later period is 

 reached in the development of the shell. 



For the plicate species nearly the full number of plica- 

 tions appear simultaneously, as in Camarotoecliia indianensis, 

 C. acinus, and Rhynchotreta cuneata, or they are introduced 

 in pairs, as upon Camarotoecliia Whitii, ?Q. neglecta, and 

 Homceospira sohrina. 



The striae of Leptcena rhomboidalis are developed to the 

 full capacity of the marginal area as soon as the first growth- 

 line is completed. The number is increased in three distinct 

 manners: (a) by division, (b) by intercalation, and (c) by 

 addition at the cardinal angles. Some species present all 

 three of these, while others add to their strice or plications by 

 any one or two of the modes. 



The concentric ornamentation in such species as Spirifer 

 crispus and OrtJiothetes suhplanus appears early in the growth 

 of the embryo, and continues to be repeated without varia- 

 tion, except in Leptcena rhomboidalis and allied forms, which 

 develop, during the last stage of growth, a geniculated cur- 

 tain without concentric undulations. 



Varieties and Abnormalities. — Varieties usually begin to 

 express themselves early in the development of the shell, 

 and the divergence from the normal form rapidly increases as 

 maturity approaches. Several of the species represented by 

 abundant material are readily separable into three distinct 

 groups of forms, (a) Ion ff form, (b) normal form, and (c) broad 

 form. The history of each may be clearly traced, and they 

 usually are found to unite with the line of the normal form 

 (5) several removes from the initial member of the series. 

 Sometimes the varieties do not reach the adult dimensions of 

 the normal shell and may be considered as varietal dwarfs. 



The representation of varietal and of certain abnormal con- 

 ditions by complete series of fossil specimens shows that in 

 these directions there was a distinct developmental tendency, 

 or genetic impulse, independent of normal growth. 



Senility is always expressed by the thickening of the shell 



