COCKERELL : ON HELIX SUBPLICATA, SOWBRBY. 195 



Plehecula. 



P. punctulata, Sowerby (Helicomela). Median teeth with small 



side cusps, their upper margin horizontal, not rising above lateral 



notch ; first laterals with strong triangular ectocones ; marginals 



with outer cusp bifid and inner simple, thus wholly unlike Idiomela. 



Geomitra. 



G. hicarinata, Sowerby (Hystricella). Jaw with ten broad, flattened 

 ribs, with narrow intervals between ; median teeth with small 

 lateral cusps ; first laterals with large ectocones ; marginals with 

 inner cusp strongly bifid, outer also bifid, all the points sharp, wholly 

 unlike Idiomela. G. echinulata, Lowe, has teeth like hicarinata. 



G.polymGTpha var. discina, Lowe (Discula). Jaw clear yellowish, 

 v/ith ten broad, flattened ribs, just as in bicariiiata ; median teeth 

 with distinct lateral cusps ; laterals with distinct ectocones ;; 

 marginals with long sharp inner cusp, with small and sharp lobe 

 on inner side, outer cusp bifid. 



In Pilsbry's classification, H. subplicata falls in the section or 

 group Erctella, Monts., the type of which is the Sicilian H. mazztillii, 

 Jan. The raduUa of H. muzztdlii from Palermt) has the centrals 

 with very small angular side cusps ; laterals with well-developed 

 ectocones, stronglv angulate ; marginals, very variable, three or 

 four lobed (Fig. 8). 



H. subap>erta, Ancey from Algeria has been referred to the same 

 group, but it is very distinct. The median teeth have distinct 

 cusps ; laterals with large side cusps, equally strong on each side, 

 so that they are nearly symmetrical ; marginals with inner cusp 

 strongly bifid, the lobes obtuse, outer simple and rudimentary 

 (Fig. 7). These species show little resemblance to Idiomela. 



There is reason to believe that while the present island of Porto 

 Santo contains no rocks bearing fossils older than the Miocene, it 

 rests upon an older basis, now submerged. The snail fauna seems to 

 represent the remnants of the life of this older, doubtless Mesozoic, 

 land. Idiomela may be regarded as an isolated type, related to the 

 common stem of the continental Helix, but distinct from any of the 

 living or fossil continental genera. 



The species H. subplicata is at least as old as the Pleistocene, to 

 which period the fossil specimens must be referred. West of the 

 Villa Baleira is a region in which, some 30 feet below the surface, a 

 bed of marine Pleistocene rests upon a basis of dark volcanic rock, 

 and is covered by sands containing land shells. The land shells 

 are, however, not necessarily younger than the marine beds, as the 

 sand, shifting as it does to-day, may have been blown over them. 

 At any rate, I found a specimen of Geomitra coronata, Desh., firmly 

 embedded in the dense marine deposit, mixed with the marine shells 



