262 



MOLLUSCA. 



the animal. Two doubtful species belonging to the genus 

 Umbrella have been described from the Tertiaries ; but the 

 family is otherwise unknown in the fossil condition. 



CHAPTER XXII. 



GA STEROPODA Continued. 



HETEROPODA AND PULMONIFERA. 



ORDER III. HETEROPODA OR NUCLEOBRANCHIATA : The 

 Gasteropods of this order differ from the typical members of 

 the class in being organised to lead an existence in the open 

 ocean, locomotion being effected by a fin-like tail, or by a fan- 

 shaped vertically-fiatte?ied ventral fin. They are found swim- 

 ming at or near the surface of the ocean ; and the body may be 

 completely protected by a shell, within which the animal can 

 retire, and which can be closed by an operculum. In other 

 cases, as in Carinaria (fig. 230), the body is large, and there is 



Fig. 230. Heteropoda. Carinaria, cymbium. /Proboscis; Tentacles; 

 Shell ; / Foot ; d Disc. (After Woodward.) 



Branchiae ; 



only a small shell protecting the gills and heart. In other 

 cases, again, the shell is completely wanting. The order is 

 divided into the two families of the Firolidce and Atlantidce. 

 The former of these is represented by a single species only, 

 from the Miocene Tertiary. The latter had a great develop- 

 ment in Palaeozoic seas, and is represented in the formations 

 of this period by several remarkable genera. 



FAM. i. FIROLID^ : Body large, never completely pro- 

 tected by a shell, often shell-less. Sometimes a small delicate 



