308 GEOLOGICAL BIOLOGY. 



or three directions. In the series in which the striae con- 

 tinue unbroken by imbrication there is an increase in their 

 strength until, in Carboniferous times, 

 the species of this series develop a 

 spinous extension of the surface with 

 minute tubes, extending outward from 

 the shell : these tubes are seen in a 

 few of the Devonian forms also. In 

 the race with imbricated surface, where 

 the imbrication is persistent and regu- 

 lar, the striate structure becomes entire- 

 ly obliterated in the course of time 



FIG. 06. An enlargement of the , ~ r ^ . . 



surface of Spiriferpseudolinea- (SCC figures OI O. CriSpUS, QO, Q2). In 

 tusHall. At e the test has been X . , . . . . 



partially removed, exposing the others, where imbrication is irregular, in 



tubular character of the spines . 



below the surface of the shell; at the Devonian and the Carboniferous 



a the spines are perfect ; at b, 



broken away ; at * they are rep- eras there are species with roughened 



resented as weathered, showing 

 the tubular character of the 

 spines ; and at d they are broken 



the tubular character of the surface, irregular but granular (as 5. 



ken 



of the Hamilton), and this 

 indicates a development of part, with 

 kuk,iowa. (After Hail) ' obliteration of others, of these surface- 

 reaching ends of the striae. All the modification noticed in 

 this respect is also extrinsic, and can be accounted for by 

 processes of natural selection, slowly intensifying the character 

 with repeated generation. 



F and G. Plication of Surf ace and Median Fold and Sinus. 

 The next character to be noted is that of the plication of 

 the surface ; each species is pretty constant in the extent to 

 which this modification reached, but in the early forms of the 

 Niagara formation there is extreme range of variation, not 

 only in the whole set, but in the species which are, in other 

 respects, less variable. 



Spirifer plicatella, variety radiatus (Fig. 83), is generally 

 lacking in plications ; but in Europe there are specimens (gen- 

 erally associated with the others) in which the plications are 

 seen on the margin of the adults (see Figs. 84, 85); a few 

 plications appearing on each side of the medial fold. In Amer- 

 ica the plicated form is called 5. Niagarensis (Fig. 87), and is 

 uniformly plicated to the beak. In the series vS. crispus and 

 5. sulcatus (Figs. 88-93) we find the same variability, speci- 



