248 AN INTRODUCTION TO THE STUDY OF FOSSILS 



is no distinct head. A mantle sur- 

 rounds the body and secretes the shell. 



There is a general symmetry of the 

 body organs resulting from a second 

 torsion of 180° in a direction opposed 

 to that of the original torsion of other 

 gastropods ; the primitive torsion ap- 

 pears in the course of the animal's de- 

 velopment. This secondary symmetry 

 is most manifest in the nervous system. 

 The visceral connectives are not crossed 

 as they are in Busycon, and the nerve 

 elements are concentrated in the an- 

 terior region, around the oesophagus. 



The two sexes are united in one 

 individual. 



Clio is one of the Pteropoda, — a 

 group formerly differentiated among 

 the Gastropoda, but now usually broken 

 up and scattered among various divi- 

 sions of the tectibranchs, — a sub-order 

 of the Opisthobranchia. They were 

 formerly segregated on the basis of their 

 common possession of a foot trans- 

 formed into two wing-like fins and of 

 their lack of a distinct head. 



r.((^ f. ^.., r^^ef.r;^. o^H I- What charactcrs do Clio and the 



01 foot ; post., posterior end r i i i i -ri 



of shell ; rep., reproductive Other members of the old class Ptero- 

 poda possess in common which distin- 

 guish them from the rest of the 

 gastropods ? 



Fig. 108. — The gastropod, 

 Clio acicula, ventral view; 

 this has a universal dis- 

 tribution. Much enlarged. 

 ant., anterior margin of 

 shell ; g.o., genital opening ; 

 /./., lateral fin, — the wing- 

 like lateral lobe of the foot ; 

 li., liver ; p.f., posterior fin, 

 — the median posterior lobe 



gland; 5/., stomach. (Re- 

 drawn from Lankester's 

 Zoology after Souleyet.) 



Tentaculites (Fig. 109). Ordovician-Devonian. 



The shell is thick-walled, conical, and tapering posteriorly to 

 an acute apex. The surface of the shell is banded with raised 

 parallel rings. 



