4 o8 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



f 



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, //;/. 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 



