540 



MOLLUSCA 



PHYLUM VI 



Lacuna Tiirton (Fig. 927). Like the last, biit thiii, small, with an excavated pillar. 

 Tertiary and Recent. 



Fi(f. 926. 



Littorina litorea (Linn.). 

 Post-Pleistocene ; Isle of 

 Skaptö. 



Fig. 927. 



Lacuna (?) basterotina 

 Bronn. Miocene ; Stein- 

 abrunn, near Vienna. 



Fig. 92 



Fossarus costatus Brocchi. Pliocene : 

 Limite, Tuscany. 



Lacunella Desli. Eocene. Litiopa Rang ; Planaxis Lam. ; Quoijia Desh. Tertiary 

 and Recent. The families Litiopidae and Planaxidae are usually recognised. 



The genns Fossarus Phil. (Fig. 928) forins, according to Fischer, a separate family, 

 Fossaridae. It occiirs in the late Tertiary and Recent. 



Family 3. Cyclostomatidae Menke. 



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

 with epidermis. Äperture 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 apjDroach the Littorinidae very closely, which latter forms also 

 have the gill much reduced. The shell habit is excessively variable. Tliere are more 

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



Fig. 929. 



Cydostoma hi- 

 sulcatum Zieten. 

 Miocene ; Ermin- 

 gen, near Ulm, 

 Würtemberg. 



Fig. 930. 



Pomatias labellum 

 (Thoraas). Helix 

 Beds (Upper Oligo- 

 cene); Hochheim, 

 near Wiesbaden. 



Fig. 931. 



Cydotus exaratus Sandb. 

 Upper Eocene ; Pugnello, 

 Italy. Shell and operculum 

 (after Sandberger). 



Fig. 932. 



Strophostoma anom- 

 phala Capellini. Oligo- 

 cene ; Arnegg, near 

 Ulm, Würtemberg. 



of tliese are tropica! Fossil forms are found in fresh-water deposits as old as the 

 Middle Cretaceous. 



Gyclostomus Montf. (Fig. 929). Turbinate, with calcareous spiral operculum. 

 Tertiary and Recent. 



Otopoma, Tudora Gray. Tertiary and Recent. 



Megalomastoma Guild. Turbinate to chrysalis-shaped, usually smooth. Peristome 

 with thick margins ; outer lip reflected. Operculum horny. Cretaceous to Recent. 

 M. mumia (Lamarck). 



Pomatias Studer (Fig. 930). Turreted, transversely striated, with reflected 

 margins and calcareous operculum. Tertiary to Recent ; palearctic. 



Leptopoma Pfeiff.; Gijclophorus Montf.; Graspedopoma Pfeift'.; Gyclotus Guilding 

 (Fig. 931), etc. Upper Cretaceous. These genera are considered to form a distinct 

 family, Gyclophoridae. Strophostoma Desh. (Fig. 932). Upper Cretaceous to Miocene. 



