GASTEROPODA. 253 



termination of these fossils by the shells alone is attended with 

 great difficulties. 



FAM. 10. MELANIAD^E. Shell spiral, turreted; aperture 

 often channelled or notched in front ; outer lip acute. Oper- 

 culum horny and spiral. Many fossil shells have been referred 

 to the Melaniada, but it is probable that most of these belong 

 to the Palaeozoic genus Loxonema and the Mesozoic Chem- 

 nitzia. The true Melanice do not appear to have commenced 

 their existence till the Eocene Tertiary. All the living species 

 inhabit fresh water, generally in the warmer parts of the world ; 

 and it is probable that all the fossil species occur only in fluvia- 

 tile and lacustrine deposits. 



FAM. ii. TURRITELLID^E. Shell tubular or spiral, often 

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

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

 plume single. The Turritellidcz are not known to have existed 

 in the Palaeozoic period ; but they appear to commence about 

 the middle of the Jurassic period, abounding in the Tertiaries, 

 and attaining their maximum in existing seas. The chief 

 fossil genera are Turritella, Vermetus, and Scalaria. 



In Turritella (fig. 217) the shell is turreted. many-whorled, 

 and spirally striated ; the aperture is 

 small and rounded, and the peristome 

 thin. Species of Turritella have been 

 described from the Palaeozoic and older 

 Mesozoic formations, but almost cer- 

 tainly belong to the genera Murchisonia 

 and Loxonema. The genus is for the 

 first time represented with certainty in 

 the Lower Cretaceous Rocks (Neo- 

 comian), and many fossil species are 

 found in the Tertiaries. 



The genus Vermetus comprises tubu- 

 lar shells, the chief interest of which 



,-, 11 -i i .1 Fig. 217. Turntella angu- 



is the strong resemblance which they ia ta . Neocomian. 



show to the Annelidous genus Serpula. 



The shell is attached, and though regularly spiral when young, 

 is always irregular in its growth when adult. The fossil species 

 are best distinguished from Serpula by the fact that the tube 

 is repeatedly partitioned off by calcareous septa, as the animal 

 grows. It is, however, often a matter of extreme difficulty to 

 determine whether a given specimen be a Vermetus or a Ser- 

 pula. Fossil Vermeti are known from the Lower Cretaceous 

 upwards. 



The genus Scalaria comprises the Wentle-traps, in which 



