8lO DIVISIONS OF THE GASTROPODA. 



tive thickness and solidity of the shell, together with its remarkable 

 minute structure, must be considered as points which militate against 

 a reference of the genus to the Pteropods. On the other hand, its 

 free habit of existence, its general form, its mode of occurrence, and 

 its occasional possession of a distinct initial chamber or embryonic 

 shell would support the view that it belongs to the Pteropoda. The 

 occasional presence of curved internal septa is a point of small 

 weight, one way or the other, since such transverse partitions are 

 not absolutely unknown in recent Pteropods (Cuvieria). Still less 

 stress can be laid upon the argument that Tentaculites can hardly be 

 regarded as a Pteropod, seeing that the normal forms of the Ptero- 

 poda do not make their appearance till the Tertiary rocks are 

 reached. Any weight that might be supposed to be carried by such 

 an argument is counterbalanced by the fact that in Styliola fissurella 

 and its allies we have Palaeozoic forms which have not been shown to 

 differ materially from existing types of Pteropods. By some palae- 

 ontologists Tentaculites is regarded as belonging to the Tubicolar 

 Annelides ; and casts of the shell of Tentaculites are undoubtedly 

 very similar to casts of the tube of Cornulites. The microscopic 

 structure of the tube of Tentaculites is, however, widely different 

 from that of Cornulites, while the tube shows no signs of having 

 been at any time of life attached to any foreign body. It is, how- 

 ever, quite possible that some of the forms which have been referred 

 to Tentaculites are really Annelidan in nature. Thus, Hall has 

 pointed out that all the forms from the Ordovician rocks of North 

 America which have been placed in Tentaculites are curved and are 

 longitudinally striated ; and he regards these, therefore, as probably 

 truly belonging to Cornulites or to some related Annelidan genus. 



Sub-Class II. Pulmogastropoda. 



The Pulmonate Gastropods are essentially distinguished by the 

 fact that the breathing-organ is a pulmonary chamber, formed by an 

 inflection of the mantle, to which air is directly admitted by an ex- 

 ternal aperture ; while the sexes are united in the same individual. 



The most typical members of this division of Gastropods, such as 

 the Land- snails and Slugs, are terrestrial in habit. Others, like the 

 Limnceidce, inhabit fresh waters, and either come to the surface to 

 obtain air, or, in some cases, have the power of using the lung- 

 chamber as an organ of aquatic respiration. A few forms only 

 (Siphonariidce, Gadiniidce) are inhabitants of salt water. 



The condition of the shell varies much, a few forms (e.g., On- 

 cidium) being wholly without a shell. In the Slugs, a shell is 

 present, but is of small size, and is concealed within the mantle. 

 In the ordinary Snails, again, and in the Pond-snails, there is a well- 



