94 MEMOIRS OF THE STATE MUSEUM 



The observations by Morse are corroborated by those of Dali * on 

 Terebratula cubensis. 



Surface Ornaments, Nearly all the observations upon initial shells 

 or upon that portion situated at the apex of the beak of more advanced 

 stages and representing the initial shell, seem to warrant the assertion 

 that the surface ornaments 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 plications appear 

 simultaneously, as in Rhynchonella indianensis, R. acifius, and Rhynchotreta 

 ctincata, or they are introduced in pairs, as upon Rhynchonella whitii, 

 ? R. neglecta, and Retzia sobrina. 



The striae of Strophomena 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 striae or 

 plications by any one or two of the modes. 



The concentric ornamentation in such species as Spirifer crispus and 

 Streptorhynchis subplanum appears early in the growth of the embryo, and 

 continues to be repeated without variation, except in Strophomena rhom- 

 boidalis and allied forms, which develop, during the last stage of growth, 

 a geniculated curtain 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) long form, (b) normal form, and' 

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



♦Proceedings of the Academy of Natural Sciences of Philadelphia, 1877, pt- '' ; Report on 

 the Brachiopoda of Alaska and the adjacent Shores of North-west America, p. 155. 



