GASTEROPODA. 263 



hyaline shell, placed on the back, protecting the gills. The 

 only genus of this family which is represented in a fossil state 

 is Carinaria (fig. 230), a single species of which has been found 

 in deposits of Tertiary age (Miocene). 



FAM. 2. ATLANTID^E : Animal furnished with a well-de- 

 veloped shell, into which it can retire. Shell symmetrical, 

 discoidal, destitute of septa, often provided with an operculum. 

 This family is represented by the genera Bellerophon, Maclurea, 

 Cyrtolites, Ecculiomphalus, and Porcellia, most of which are 

 exclusively Palaeozoic, whilst the others are mainly so. 



In the genus Bellerophon (fig. 231) the shell is symmetrical, 

 convoluted, the coils of the shell usually lying in one plane. 



Fig. 231. Bellerophon Argo (Billings), a Front view ; b side view. Lower Silurian. 



The whorls are few, smooth or sculptured, and there is a 

 dorsal keel along the convex margin of the shell. The aper- 

 ture is often more or less expanded, and is in most instances 

 emarginate or deeply notched on the dorsal side. The genus 

 ranges from the Lower Silurian to the Carboniferous. The 

 Bellerophina of the Gault (Upper Cretaceous) is doubtfully 

 allied to Bellerophon, and may belong to -the Pteropoda. 



The genus Maclurea is very remarkable in its structure, and 

 all the known species are entirely confined to the Lower 

 Silurian Rocks. The shell (fig. 232) in this singular genus is 

 " discoidal, few-whorled, longitudinally grooved at the back, 

 and slightly rugose with lines of growth ; dextral side convex, 

 deeply and narrowly perforated ; left side flat, exposing the 

 inner whorls ; operculum sinistrally sub-spiral, solid, with two 

 internal projections, one of them beneath the nucleus, very 

 thick and rugose" (Woodward). Maclurea has been variously 

 regarded as " dextral " or " sinistral ; " but the probabilities are 

 in favour of the view that it is truly dextral. In this case, the 

 flat side of the shell is the umbilicus, and the spire must be 

 regarded as sunk below the general surface of the shell. (On 

 this view the specimen figured at b, fig. 232, is represented up- 

 side down.) The species of Maclurea occur chiefly at the base 



