786 



DIVISIONS OF THE GASTROPODA. 



Fig. 688. — Cyclostoma 

 Arnoudii. Eocene Ter- 

 tiary. 



is turreted. Both genera range from the Cretaceous period to the 

 present day. 



Family 29. Cyclostomid/e. — In this family are included terres- 

 trial Gastropods, which possess a pulmonary chamber, and which 

 have therefore been commonly included among 

 the Pulmogastropoda. The shell is of variable 

 shape, spiral, sometimes conical (fig. 688), 

 sometimes depressed or discoidal. The aper- 

 ture is nearly circular, and the peristome is 

 simple or reflected. An operculum is present, 

 and is usually more or less extensively calcified. 

 The principal genus in this family is Cyclostoma 

 (fig. 688) itself, which has been split up into a 

 number of subordinate groups. The oldest 

 forms of Cyclostoma, in the general sense, ap- 

 pear in the Cretaceous rocks, and a number of Tertiary species are 

 known, while the genus is largely represented at the present day. 

 Family 30. Aciculid/e. — This family includes small terrestrial 

 Gastropods, provided, like the preceding, with a 

 pulmonary chamber. The shell is elongated and 

 cylindrical, with a blunt apex, and a thickened 

 peristome. There is an operculum, but this is 

 never calcified. The recent genus Acicula is 

 represented in the later Tertiary deposits. 



The genus Truncatella — sometimes regarded as 

 the type of a separate family (TruncatellidcE) — com- 

 prises littoral or fresh-water Gastropods, sometimes 

 semi-terrestrial in habit, and related on the one hand 

 to Acicula, and on the other hand to the Rissoidce. 

 The nature of the breathing-organs has not been 

 clearly ascertained. The shell in Truncatella is elon- 

 gated and cylindrical, often truncated in the adult 

 state, and with an entire oval aperture. Species of 

 the genus have been detected in the Tertiary de- 

 posits, the oldest appearing in the Eocene. 



Family 31. Subulitid^. — In this family the 

 shell (fig. 689) is elongate and fusiform, with a 

 smooth surface. The aperture is long and nar- 

 row, and is distinctly canaliculated in front. This 

 family includes some Palaeozoic and Triassic Gas- 

 tropods, of which Subulites is the type, in all of 



Norfh^merka SI \ U After wnicn tne peristome i s incomplete and the shell 



Hail.) notched in front. By Lindstrom the members 



of this group are regarded as the most ancient 



representatives of the division of the Siphonostomatous Proso- 



branchs. 



Fig. 689. — Subulites 



