SUB-ORDER B PLATYPODA 459 
flattened surface beneath the suture bounded by a row of nodes. Last whorl inflated, 
smooth. Aperture anteriorly with canal - like notch; outer lip thin. Jura and 
Cretaceous. : 
Brachytrema, Morris and Lycett ; Tomocheilus, Gemm. Jura. 
Family 2. Littorinidae. Gray. 
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 the Turbinidae 
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 FIG. 876. 
genera which exhibit great similarity to Littorina, but are Turbonitella _ subcostata, 
often assigned to the above-named families, may be mentioned See Pun Ree 
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. 
Sih). Thick -shelled, 
turbinate to globose, 
smooth or spirally 
TTC, Basten O- striated, without wum- 


= TDs 
S 

Fic. 878. 

Fic. S877. 
Littorina littorea, tina, Bronn. 
Linn. sp. Post- Miocene; Steina- Be, Se bilicus. Aperture oval. 
Pleistocene ;_ Isle brunn, near Fossarus costatus, Broechi. Plio- ea Recens 
of Skapto. Vienna. cene ; Limite, Tuscany. ura to hecent. 
Lacuna, Turton 
(Fig. 878). Like the last, but with a narrow umbilical fissure. Tertiary and Recent. 
Lacunella, Desh.; Eocene. Litiopa, Rang; Planazis, Lam.; Quoyta, 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. 
Family 3. Cyclostomidae. Menke. 
Shell extremely variable in form, turbinate to discotdal, sometimes turreted, covered 
with epidermis. Aperture circular, with usually entire peristome. Operculum horny or 
calcareous, spiral. Terrestrial. Cretaceous to Recent. 
Like the pulmonate snails, the animal possesses a respiratory cavity. But in 
other respects they approach the Littorinidae 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 
