946 PRINCIPLES OF STRATIGRAPHY 



oids are represented by the living Nautilus. These two groups 

 are classed as Tetrabranchiata. The Dihranchiata are represented 

 by the living Argonauta, the Octopus, Squid, Cuttlefish, and 

 Spirula. The last is an internal loose-coiled shell with septa and 

 siphuncle. A straight-coiled ancestor, the Jurassic and Cretacic 

 Belemnites, had its shell, which was straight, protected by a heavy 

 calcareous outer guard, often cigar-shaped, and when perfect 

 showing the hollow at one end occupied by the shell. A modified 

 portion of the guard alone remains in the cuttlefish, the so-called 

 cuttlefish bone, which is embedded in the fleshy mantle of the 

 animal. 



The Pteropoda have thin transparent shells of various shapes, 

 but rarely coiled. The "foot" of the animal is divided into two wing- 

 like appendages by which these "Butterflies of the sea" keep them- 

 selves afloat on the water. The shell of the Connlariidce and Hvo- 

 lithidcr was coarser and generally rectangular in section in the 

 former and variously shaped in the latter. The Scaphopoda (Den- 

 talium, etc.) have conical, often curved, shells, open at both ends, 

 which begin as a saddle-shaped structure growing into a ring and 

 increasing in length. In the Polyplacophora (Chiton, etc.) the shell 

 is composed of several pieces arranged serially. 



Pelecypoda are rare in the Cambric but become abundant in 

 the succeeding horizons. There are about 10,000 fossil species and 

 about 5,000 recent ones. The Gastropoda are likewise sparsely 

 represented in the Cambric. They appear to be at their acme of 

 development at the present time, there being some 15,000 living 

 species, as compared with about half that number or less of fossil 

 ones. Only one cephalopod is known from the Cambric. They 

 abound in the Ordovicic, at the end of which period many races 

 died out, while new ones arose. The Ammonoidea begin in the 

 Devonic, reach their acme in the Jurassic and die out in the Cretacic. 

 The NauHloidea and Dihranchiata (the latter appearing first in the 

 Trias) have modern representatives. 



Phylum VI — Platyhelmintha, and VII — Vermes. The pla- 

 tyhelminths, or flat-worms, are soft-bodied, worm-like animals with- 

 out body cavity or coelom. They have no hard parts, and nothing is 

 known of their geological history. The great mass of animals 

 classed together as Vermes is in reality a heterogeneous assemblage, 

 many of the groups having no direct relationship with others placed 

 here. Typical worms (chaetopods) have a distinct body cavity from 

 which the enteric and digestive tracts are separated. The body is 

 divided into many similar segments, each of which, except the oral 

 one, carries on each side two bundles of bristles or set?e, a dorsal 



