GASTEROPODA. 



31 



Fig. 418. — Murclii- 

 sonia gracilis, (Hall.) 

 Lower Silurian. 



Closely allied to Flcurotomaria is the important genus 

 Murchisonia (fig. 418), whicli is exclusively confined to the 

 Palaeozoic period, ranging from the Lower Silurian to the 

 Permian. The shell in Murchisonia closely resembles that of 

 PleuTotomaria, but is usually more elongated and composed of 

 a greater number of whorls. The outer lip 

 is deeply notched, and the whorls have the 

 same band on their exterior as is present 

 in Pleurotomaria. The aperture of the 

 shell is slightly channelled in front, and 

 the surface is often variously sculptured 

 and adorned. 



Finally, we may include in the present 

 family the living genus lanthina, which in 

 some respects may be regarded as the type 

 of a distinct group. In this genus the 

 shell is spiral and turbinate, with a thin 

 translucent shell ; and it is doubtful if 

 any fossil forms can be definitely referred 

 to it. 



Fam. 17. FissuRELLiD^. — Shell conical, patelliform, with 

 a notch in the anterior margin, or a perforation at the apex, 

 which is occupied by the anal siphon. Muscular impression 

 horse-shoe-shaped, open in front. The existence of the Fis- 

 surellidce in the Palfieozoic period is open to considerable 

 doubt; but a good many fossil forms are known from the 

 Secondary and Tertiary rocks. 



The genus Fissiorella (fig. 419, a) comprises the so-called 

 " Keyhole Limpets," distinguished by having the apex of the 

 shell perforated by a larger or smaller, generally oval aper- 

 ture. Doubtful examples of the genus have been indicated 

 as occurring in the Devonian and Carboniferous ; and there 

 are a good many unequivocal species in the Secondary and 

 Tertiary rocks. In the genus Eimula (fig. 419, b), ranging 

 from the Trias to the present day, the perforation, instead of 

 being at the apex of the shell, is placed a little above the 

 anterior margin. Lastly, in Emarginula (fig. 419, c) the 

 anterior margin is furnished with a longitudinal notch or 

 slit. The species of this genus date from the Trias. 



