GASTEROPODA. 83 



globose form, with a similar narrow umbilicus on either side 

 (fig. 26, 7), wanting, however, in B. nodocarinatus of the 

 Illinois coal-beds. Sometimes the shell is thin and the aper- 

 ture expanded, like a trumpet, whilst other species are globu- 

 lar and solid; the former may have been tenanted by large 

 animals living at the surface of the open sea, the latter seem 

 to have been more adapted to protect their owners crawling 

 over the bottom, for it can scarcely be insisted that all were 

 necessarily floaters on account of their organization. The 

 species of Belleropkon are numerous in all the palaeozoic rocks, 

 and some of the smaller kinds appear to have been gregarious : 

 those with disconnected whorls have been called Cyrtolites 

 (Conrad.) The Belleropliina of d'Orbigny (fig. 27, «), is a 

 minute shell found in the gault. 



The family Firoliclce includes the Nucleobranchs in which 

 the shell is wanting, or uncalcified, or is very small com- 

 pared with the bulk of the animal. A single species of the 

 genus Garinaria, the most beautiful of the group, with a 

 hyaline shell shaped like that of the Argonaut, suspended 

 from the body, has been found in the middle tertiary of Turin. 



Normal Orders. 



In the majority of Gastropods the shell is a " spiral uni- 

 valve," the varieties of which are shewn by an immense series 

 of fossils. The most simple form of univalve shell is the cone, 

 which may be much depressed, as in the genus Umbrella, or 

 extremely elevated and contracted, as in Dentalium, or of more 

 ordinary proportions, as in the limpets {Patella). The apex of 

 the cone is always oblique and eccentric ; directed, in limpets, 

 towards the head, but in other Gastropods towards the opposite 

 extremity of the body. The spiral univalve is convoluted, 

 sometimes in the same plane, as in Planorbis, but more usually 

 in an oblique direction, as in Triton (fig. 25). The apex of the 

 shell a is formed by the nucleus, or the part which was deve- 



