302 GEOLOGICAL BIOLOGY. 



more or less plastic elements of the genus. The characters 

 of Spirifers that are of chief generic and specific value are the 

 following : 



A. The form and arrangement of the spiral appendages. 



B. The general proportions of the shell. 



C. The delthyrium, deltidium, and fissure ; their shape and 

 development. 



D. Hinge area, its length and height. 



E. Surface markings; radiating striae, fine and continuous 

 or coarse and interrupted ; including imbrication. 



F. Medial fold and sinus. 



G. Plications of surface simple fold, or many and bifur- 

 cated folds. 



H. Structure of shell fibrous or punctate. 



I. Spines, or setae, or elevations, granular or otherwise. 



K. Special development of septa, medial or deltidial. 



Whatever evolution has taken place should be expressed 

 in terms of some one or more of these characters, for these 

 constitute the differences distinguishing the several known 

 species. 



A. Spiral Appendages * So far as we know, these varia- 

 tions during the life-history of the subfamily or genus do not 

 exceed slight adjustment of position and direction of the coils 

 to the internal capacity of the shell, and variation in the num- 

 ber of the coils. Of both of these characters too few statistics 

 are at hand to enable us to base upon them any law regarding 

 the rate, or even direction, of evolution ; but the modifications 

 appear to be all easily explainable by the principle of ex- 

 trinsic evolution, i.e., adaptation to external conditions in 

 the process of ontogenesis. 



B. The General Proportions of the Shells. Taking an 

 average of the extremes of form for the whole of the hinged 

 Brachiopoda and constructing a medium form, the result 

 would be an oval shell, with hinge line shorter than the great- 

 est width, and the pedicle valve larger than the brachial, with 

 low hinge area ; a deltidium ; no fold or sinus, further than a 



* See, regarding this and other details, Paleontology of New York, vol. vm., 

 " An Introduction to the Study of Genera of Paleozoic Brachiopoda," pt. II., by 

 James Hall, assisted by John M. Clarke, 1894. 



