40 



GASTEROPODA. 



size gradually towards the mouth. Tremanotus, of the same 

 formation, has a dilated aperture, and possesses a row of 

 separate oval apertures along the dorsal side. The Bellcro- 

 phontidoi are sometimes placed among the Hcdiotidcc, and 

 the genus Porcellia, often regarded as Heteropodous, has 

 been here considered as an ally of Flcurotomaria. 



The genus Madurea is very remarkable in its structure, 

 and all the known species are entirely confined to the Lower 

 Silurian rocks. The shell (fig. 429) 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). Madurea 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 fiat side of the shell is the umbilicus, and 

 the spire must be regarded as sunk below the general sur- 

 face of the shell. (On this view the specimen figured at h, 

 fig. 429, is represented upside down.) The species of Mac- 

 hirea occur chiefly at the base of the Lower Silurian series 

 both in North America and in Scotland, occurring in some 

 localities in the greatest profusion. 



Fig. 420. — Madurea cremdata. a, Spire ; h, Front ; c, Base. Lower Silurian. 



The genus 0})Mleta (fig. 430), of the Silurian rocks, may 

 be mentioned here, though its true affinities are extremely 

 doubtful. The shell in this genus is discoidal, and very 

 closely resembles that of Euomphalus. The aperture, how- 

 ever, is stated by ]\Ir Billings to have a sinus in the lower 



