4o8 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



and it formed the floor of a true living chamber, Ic, 

 formerly occupied by the animal at the time of its 

 death and burial in the sediment of the carboniferous 

 period. Fig. 115 shows a similar fossil but with a 

 longer, although still incomplete living chamber. If 

 the external wall of shell had been preserved none of 

 these structures could be seen. Fig. 117 shows a fossil 



Temnochilus crassus, a shell 



of the same family, with this 



external wall preserved, and 



all these internal structures 



covered up. The impressed 



zone is the reentrant curve 



shown in all these figures 



and especially marked in 



^'^' "^' the lower outline of an outer 



whorl of another carboniferous species, Metacoceras 



dubium Hyatt, Fig. 118, im. z. 



" It is not necessary to go into a discussion of the 

 details of internal structures and their relations to the 

 impressed zone in this abstract, but it is essential to 

 give a general description of the morphogeny of the 

 order of nautiloids. 



"This group of chambered Cephalopods contains 

 the following classes of forms : first, straight, conical 

 shells, type Orthoceras, Fig. 119, No. i; second, curved 

 cones, Cyrtoceras, Fig. 119, No. 2 ; third loosely coiled, 

 open whorled cones, do., No. 3; fourth, coiled cones 

 with the whorls more or less enveloping, do., No. 5. 

 The fourth and fifth forms are usually included in 

 the old genus. Nautilus. Practically, it is better to 

 designate the first class as orthoceran, the second as 

 cyrtoceran, the third as gyroceran, and the fourth and 

 fifth as nautilian. In tracing genetic series through 



