KEPORT OF THE STATE GEOLOGIST. 479 



of the animal, and is therefore more highly specialized and more 



susceptible to variation in its external form. 



Cardinal area.— In many of the dentiferous or articulate 



brachiopods a conspicuous feature of the shell is the specialization 

 of the posterior portion of the pedicle valve as a vertical or 

 curved triangular area. The same character is sometimes present 



Fig 49. - Cardinal .'area Figs. 50 and Kl.-Qrthothetes arctostriata ; showing cardinal 



of sc, indium msujnc]; area and de i t idium. 



enlarged 



on the brachial valve, though less highly developed. In genera,, 

 which have a straight or extended hinge-line, this area "attains 

 its most conspicuous development. Tt does not bear the orna- 

 mentation of the rest of the external surface, but is smooth, or 

 with horizontal lines of growth crossed by vertical lines' of 

 structure. 



It has already been suggested that this area probably origi- 

 nated from the close attachment of the animal by its pedicle, 



Fig. 52.— Spirifer granulosus ; showing the cardinal area. 



which obstructed the outward growth of the valves. The inti- 

 mate structure of this portion of the shell does not, however 

 differ materially from that of the remainder. In forms which 

 were evidently suspended freely throughout their later existence, 

 by a moderately long arm, this area does not exist (Terebratula' 

 Khynchonella, etc.), and this may be due to the fact that the 

 posterior growth of the valves was unobstructed by close adhe- 



31 



