7C.3 



G0tt*rl rhtjri*t*r* of r*<pbfclopu4&.) 



Owen. It represents the shell as sawn hori/.ontally through its center or along the 

 plane in which it is coiled, with the entire animal lying in the body-chamber; shows 

 the air-chambers, septa and sipho, and the various external parts of the animal. 

 Though this is a coiled shell, its structural characters are not different from many 

 of th> forms here discussed in which this shell is straight or but slightly curved. 



QBNKKAL CHAKACTBBS OK THK LOWBR SILURIAN CEPHALOPODH IIKKK DKHOKIHKD. 



We have observed that the Cephalopoda met with in the early Silurian faunas 

 are mainly of primitive types of structure. Their predecessors existed in faunas 

 before the Silurian but their remains are of infrequent occurrence, and hence our 

 knowledge of them is very restricted. With the opening of the Silurian certain 

 progressed generic types, such as Orthoceras and the shells which must still be 

 referred to Cyrtoceras, became fixed or static in their traits and were continued 

 thereafter for long periods without essential modification. 



Two structural features in these Silurian nautiloids are especially significant 

 and invite brief attention. 



1. The form of the shell. The straight, elongated shell or longicone exemplified 

 in Orthoceras, Cameroceras and Adinoceras, is the prevailing type. It is known from 



Fig. 2. An Orthoftrfu represented u rertlrally sectioned for a portion of IU 

 C. body-chamber; r. alr-chambera; x. septa; *. alpbo. 



the study of some of the later longicones that these shells, from their primitive 

 formation onward through all intermediate phases to maturity, have maintained 

 the straight mode of growth, and we may therefrom infer that such shells have 

 been derived from ancestors whose shell was also straight. The formation of such 



