PLASTICITY AND PERMANENCY OF CHARACTERS. 



303 



tendency to lengthen the central part of the ventral and 

 shorten the central part of brachial valve. Both valves would 

 be convex, but slightly so. Atrypa reticularis is not far 

 from such a medium form of an articulate Brachiopod. The 

 Spirifers vary in the following directions in respect to these 

 characters : The pedicle valve may be greatly developed 

 about the beak, forming considerable contrast between the 

 two valves. This variation is noted in species of the earliest 

 stage, in individuals of most species when contrasted, and in 

 different stages of growth of the same individual. The varia- 

 tion is most noticeable among species which are abundani 



Fig. 82. — Variations in form of Sf>irifer Verneuili. (After Gosselet.) A, outline of the form 

 Cylindriciy from the upper Frasnien ; 6, form Hemicycli, from the Frasnien ; C, form Ol'O- 

 vatiy from the Famennien ; D, cardinal view of form Etongati, Famennien ; E^ cardinal of 

 extreme of he»iicycli form, from the Frasnien ; _/"= fold of the brachial valve ; b — apex of 

 the beak of pedicle valve ; li = delthyrium ; a — cardinal area ; e = lateral extremity of the 

 cardinal area. 



and of wide range, and rare or local species are generally less 

 variable in this particular (Fig. 82).* In size there is consider- 

 able variation, but most of the species of the Silurian are 

 small for the genus, though in this respect perhaps the 



* Fig. 82 illustrates some of the conspicuous differences in form assumed 

 by the Spirifers. The variations are further interesting as occurring all 

 on the same species, and appearing on specimens selected from the same 

 geological province, from strata differing a little in age, but all from the upper 

 half of the Devonian of northern Europe. Similar specimens have been seen 

 in the corresponding rocks of New York State (see Am. Jour. Sd., vol. XLix, 

 P- 473)- 



