196 PKOCEEBrNGS OF THE MALACOLOGICAL SOCIETY. 



or the apex perforated (Fissurcllidaj), are probably of equal antiquity 

 with Patella. 



The rieurotomariidfc, represented to-day by Pleurotomaria and 

 Scissurella, present a group of the highest interest to the Malacolo- 

 gist, as affording a most striking example of the persistence of a very 

 ancient form down to the present time. The members of this peculiar 

 family of shells are all marked by having the outer lip perforated or 

 slit, more or less deeply. Of the genus Pleurotomaria four species 

 only are known to survive at the present day (namely, P. Rumphii, 

 P. Bei/ric/ni, P. Quoi/atia. and P. Adansoniana), but there arc at least 

 1160 species known occurring fossil from the Lower Cambrian to the 

 newest Tertiary species. Closely related to Pleurotomaria are such 

 forms as Scalites, Rhaphistoma, Ilelicotoma, Brihnella, Luciella, Catan- 

 tosfoma, and Polytremaria, all palteozoic genera, provided witli slits or 

 perforations. To these we may add JIurcIiisonia, a turrited shell, but 

 having the slit-band around the whorls and the fissured lip. Of this 

 genus there are upwards of 50 fossil species, ranging from the Lower 

 Silurian to the Permian. 



Trochotoma has 30 species fossil, ranging from the Lias to the Coral 

 Rag. Its lip has a single perforation near the margin. Leveillia is 

 sometimes classed with the Bellerophontidse, but is really referable to 

 the Pleurotomariidae. The shell is discoidal, many-whorled ; outer 

 lip having a deep slit, and the whorls having a well-marked band 

 running along the centre of the dorsal line. 



Placed next to the Pleurotomariidae are the Bellerophontidae, with 

 BelleropJion and Trematonoius, etc. In Bellerophon the aperture is 

 expanded, sinuated, and deeply notched on the dorsal line. There 

 are 70 species fossil, ranging from Lower Silurian to Carboniferous. 

 Trematonotus is not unlike Bellerophon in form, but instead of a 

 simple notch it has a series of perforations which are successively 

 obliterated. Several species have been described from the Silurian 

 and Devonian of North America, Sweden, England, and Bohemia.^ 



Near to Pleurotomaria there is another genus, which like it is also a 

 nacreous shell, but with perforations, not a long slit, viz., Haliotis. 

 About 75 species are recorded living ; they are veiy widely dis- 

 tributed. Four species occur fossil from the Maestricht Chalk and 

 the Miocene of Malta. 



The same difficulty prevails with regard to the Mollusca that has 

 l)ecn met with in other groups, namely, to trace back their origin to 

 a Primitive Form. 



As regards the Gastropoda, we meet with small Limpet-like shells 

 in the earliest rocks (Lower Cambrian) — Stenotheca, Tri/blidium, and 

 Scenella, and also others with a spiral nucleus, as Platyceras [Capulus), 

 Pileopsis, and even small species of Pleurotomaria. With these are 

 associated Pteropod shells referred to Hyolithes and Salterella, etc. 

 It is not, however, clear whether any one of these can be considered 

 as representing the primitive Gastropod. 



With reference to the Lamellibranchiata, such early genera as 



' See Geol. Ma<j. 1885, p. '6d, aud 1890, pp. 337 and 525. 



