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 r r c^ • ^ _\t 



sur{s,c^ oi Spiri/er ps.-udolinea- (sCe hgUrCS Ot vS . Cl'lSptlS, QO, <^2). In 

 /Mi Hall. At f the test has been ...... , . 



partially removed, exposing the Otliei'S, whcrC imbnCatlOU IS UTegUlar, 111 

 tubular character of the spines , -^^ . 1 1 /^ 1 • r 



below the surface of the shell; at the Dcvonian and thc Larbonitei'ous 



a the spines are perfect ; at <^, 



broken away ; at r they are rep- eras tlicrc are specics witli rougheued 



resented as weathered, showing ^ ° 



the tubular character of the surfacc, irregular but grauular (as 5. 



spmes ; and at li they are broken ' o o \ 



close to the edge of the lamellae. crramiHfcrus of the Hamilton), and this 



and show the opening of the tubes <-> -> J' 



in the ends. The specimen is jndicates a development of part, with 



from the Keokuk limestone, Keo- I 1 ' 



kuk, Iowa. (After Hall) obliteration of othcrs, of thcse surfacc- 



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 Surface 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 plicatellay variety radiatus (Fig. 83), is generally 

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

 erally associated with the others) in w^liich 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 S. erispus and 

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



