CEPHALOPODA. 763 
General characters of cephalopods.] 
Owen. It represents the shell as sawn horizontally 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 the forms here discussed in which this shell is straight or but slightly curved. 
GENERAL CHARACTERS OF THE LOWER SILURIAN CEPHALOPODS HERE DESCRIBED. 
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 Actinoceras, is the prevailing type. It is known from 
Fig. 2.—An Orthoceras represented as vertically sectioned for a portion of its length. 
C. body-chamber; c. air-chambers; x. septa; s. sipho. 
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 
