SUB-ORDKK I! 



PLATYPODA 



459 



surface beneath tin* suture bounded by a row of nodes. Last whorl inflated, 

 smooth. Aperture anteriorly with canal -like notch; outer lip thin. .Jura and 

 ( 'rrtaivnus. 



Brachytrema, Morris and Lycett ; Tomocheilus, Gemni. Jura. 



FIG. 876. 

 Turbonitella subcostata, 



Family Littorinidae. (iray. 



Shell turbinate, usually smooth or spirally ornamented, without nacreous layer. 

 Aperture rounded ; outer lip sharp. Operculum horny, paucispiral. Marine. Ordo- 

 vician to Recent. 



Fossil shells of this family are distinguished solely from those of tin- 7W/,/',i/V/"r 



and Trochidae by the absence of a pearly layer. The animal, 



however, differs radically. The heart has but one auricle in 



the Littorinidae, two in the Turbinidae and Trochidae. The 



radula in the last -named groups is rhipidoglossate ; in the 



present family it is taenioglossate. The differences in essential 



structure are thus seen to be considerable ; yet the shells when 



fossilised are so similar, it can scarcely be doubted that the 



so-called Palaeozoic Littorinidae are in many cases very closely 



related to genera referred to the Turbinidae and Trochidae. 



The limits of these families are therefore very uncertain, so 



far as Palaeozoic forms are concerned. Among the extinct 



genera which exhibit great similarity to Littorina, but are 



often assigned to the above-named families, may be mentioned 



the following : Holopea, Hall ; Ordovician to Devonian. 



Turbonitella, de Koninck (Fig. 876) ; Devonian and Carboniferous. Portlockia, 



Turbinilopsis, and Rhab- 

 dopleura, de Koninck ; 

 from the Carboniferous. 

 Lacunina, Kittl ; Trias. 

 Littorina, Fer. (Fig. 

 877). Thick -shelled, 

 turbiuate to globose, 

 smooth or spirally 

 striated, without um- 

 bilicus. Aperture oval. 

 Jura to Recent. 



Lacuna, Turton 



(Fig. 878). Like the last, but with a narrow umbilical fissure. Tertiary and Recent. 

 Lacunella, Desh. ; Eocene. Litiopa, Rang ; Planaxis, Lam. ; Quoyia, Desh. ; 



Tertiary and Recent. The families Litopidae and Planaxidae are usually recognised. 

 The genus Fossarus, Phil. (Fig. 879), forms, according to Fischer, a separate family, 



Fossaridae. It occurs in the late Tertiary and Recent. 



FIG. 877. 



Littorina littorea, 

 Linn. sp. Post- 

 Pleistocene ; Isle 

 of Skapto. 



FIG. 878. 



Lacuna Bastero- 

 tina, Bronn. 

 Miocene ; Steina- 

 brunn, near 

 Vienna. 



Fossarus costatus, Brocchi. 

 cene ; Liinite, Tuscany. 



Plio- 



Family 3. Cyclostomidae. Menke. 



Shell extremely variable in form, turbinate to discoidal, sometimes turreted, covered 

 with epidermis. Aperture circular, with usually entire peristome. Operculum horny or 

 calcareous, spiral. Terrestrial. Cretaceous to Recent. 



Like the pulmonate snails, tl^fciimal possesses a respiratory cavity. But in 

 other respects they approach the L^^nnidae very closely, which latter forms also 

 have the gill much reduced. The shell Habit is excessively variable. There are more 

 than 600 recent species distributed throughout all parts of the globe, but the majority 



