270 MANUAL OF THE MOLLTTSCA. 



7 fathoms water off the Orkneys, and in deep water east of tli3 

 Zetland Isles. 



Fossil, 4 species. Tertiary — . Britain Sicily. 



Plettrotomaiiia, Defrance 



Etymology, pleura, side, and tome, notch. 

 Type, P. anglica, PL X., Fig. 24. 



Shell trochiform, solid, few-whorled, with the surface variously 

 ornamented ; aperture sub-quadrate, with a deep slit in its 

 outer margin. The part of the slit which has been progressiyely 

 filled up forms a band round the whorls. 



Bistrihution, 2 species. One occurs in deep water in West 

 Indian seas. 



Fossil, 400 species. Lower Silurian — Chalk. North America, 

 Europe, Australia. Specimens from clay strata retain their 

 nacreous inner layers ; those from the chalk and limestones 

 have lost them, or they are replaced by crystalline spar. 

 Pleurotomarise with wavy bands of colour have been obtained 

 in the carb. limestone of Lancashire. In this extensive group 

 there are some species which rival the living turbines in magni- 

 tude and solidity, whilst others are as frail as ianthina. 

 Sub-genera. Scalites, Conrad, L. Silurian, New York. 

 Shell thin ; whorls angular, flat above (tabulated), 8 species. 

 L. Silurian — Carb. 



Polytremaria, D'Orbigny, is founded on P. catenata 

 (Koninck), in which the margins of the slit are wavy, converting 

 it into a series of perforations. 



Catantostoma (clathratum) Sandberger, 1842. Shell like 

 Pleurotomaria ; last whorl deflected, peristome incomplete, 

 slightly varicose, irregular. Fossil, Devonian, Eifel. 



Raphistoma (angulata). Hall. L. Silurian, United States, 

 Canada. Shell depressed, outer lip sinuated. In R. compacta 

 (Salter) the spire is sunk and basin-shaped, the umbilical side 

 flat, and the last whorl a little disunited. 



MmcHisoNiA, D'Archiac. 



Etymology, named in honour of Sir Eoderick I. Murchison. 



Type, M. bilineata, PL X., Fig. 25. 



Shell elongated, many-whorled ; whorls variously sculptured, 

 and zoned like pleiu-otomaria ; aperture slightly channeled in 

 front ; outer lip deeply notched. 



The murchisonice are characteristic fossils of the palaeozoic 



