GASTEEOPODA. 1 9 



spindle-sliaped, witli a turreted spire, the outer lip of the 

 adult being greatly expanded and lobed. Owing to the 

 development of a well-marked anterior canal, and often of 

 an equally conspicuous posterior tube as well, the shell be- 

 comes distinctly " siphonostomatous," and the genus should 

 probably be placed in the immediate ^dcinity of Pteroceras 

 and Rostdlaria. The entire group makes its first appearance 

 in the Jurassic period, attained its maximum in the Creta- 

 ceous, decreased in number in the Tertiary, and is repre- 

 sented by a few forms at the present day. 



Fam. 10. Melaniad^e. — Shell spiral, turreted; aperture 

 often channelled or notched in front ; outer lip acute. 

 Operculum horny and spiral. Many fossil shells have been 

 referred to the MclaniadcB, but it is probable that most of 

 these belong to the Palaeozoic genus Loxonema and the 

 Mesozoic Chemnitzia. Various forms of Melania have been 

 described from the later Secondary rocks, and the genus is 

 well represented in the Tertiary period, as are also the 

 allied Mclanopsis and Gyrotoma. All the living species in- 

 habit fresh water, generally in the warmer parts of the 

 world ; and it is probable that all the fossil species occur 

 only in fluviatile and lacustrine deposits. 



Fam. 11, TUREITELLID.^. — Shell tubular or spiral, often 

 turreted; upper part partitioned off; aperture simple. Oper- 

 culum horny, many-whorled. Foot very short. Branchial 

 plume single. The TurritcUidce are not known to have 

 existed in the Palaeozoic period ; but they appear to com- 

 mence about the middle of the Jurassic period, abounding in 

 the Tertiaries, and attaining their maximum in existing seas. 

 The chief fossil genera are Turritdla, Vermehcs, and Scalaria. 



In Turritdla (fig. 402) the shell is turreted, many- 

 whorled, and spirally striated ; the aperture is small and 

 rounded, and the peristome thin. Species of Turritdla 

 have been described from the Palaeozoic and older Mesozoic 

 formations, but almost certainly belong to the genera Mur- 

 diisonia and Loxonema. The genus is for the 'first time 

 represented with certainty in the Lower Cretaceous rocks 

 (iSTeocomian), and many fossil species are found in the 

 Tertiaries. 



