258 



MOLLUSCA. 



to the student of the older rocks, as it is exclusively confined 

 to the Palaeozoic period, ranging from the Lower Silurian to 

 the Permian. The shell in Murchisonia closely resembles 

 that of Pleurotomaria, but is usually more elongated and com- 

 posed 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. 



We may place here, provisionally, the Palaeozoic genus 

 Holopea (fig. 225), the exact affinities of which are doubtful. 



The shell in this genus is 

 spiral, the aperture oval, 

 and the outer lip sinuated 

 near the base. The genus 

 has been compared to the 

 violet-snail (lanthina) of 

 the Atlantic, in which 

 case its place should be 

 here; but its true posi- 

 tion is altogether uncer- 

 tain. It is exclusively 

 Silurian in its range ; and 

 it is probable that the 

 genus Platyceras should 

 be united with it, in which 

 case the vertical range 

 will be extended at any 

 rate to the Carboniferous. 

 FAM. 17. FISSURELLID^E : 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- 

 surellidcz in the Palaeozoic period is open to considerable 

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

 Secondary and Tertiary Rocks. 



The genus Fissurella comprises the so-called " Keyhole 

 Limpets," distinguished by having the apex of the shell per- 

 forated by a larger or smaller, generally oval aperture. Doubt- 

 ful examples of the genus have been indicated as occurring in 

 the Devonian and Carboniferous ; but there are a good many 

 unequivocal species in the Secondary and Tertiary Rocks. In 

 the Oolitic genus Rimula the perforation, instead of being at 

 the apex of the shell, is placed a little above the anterior 

 margin. Lastly, in Emarginula the anterior margin is fur- 



Fig. 224. Murcki- 

 sonia g racilis ( H al 1) . 

 Lower Silurian. 



Fig. 225. Hotopea 

 Guelphensis (Billings). 

 Middle Silurian. 



