PLEUROTOMARIA. 



TESTA turbinata, spiralis, apertura quadrato- 

 subrotunda; labio externo aeuto, emargina- 

 tione profundi supernfe, prope suturam posita. 



Of the genera distinguished by a more or less deep fissure 

 or notch in the upper part of the outer lip, the Pleoroto- 

 maria of De France (including, in our opinion, the Scissu- 

 rella of D'Orbigny) has no canal, consequently it appears, 

 as far as a judgment can be formed from the shell alone 

 to belong to the family of the Turbinidce or Trochidce, 

 This Genus, if we except some very minute recent species 

 to which D'Orbigny has given the generic appellation of 

 Scissurella, is only found in a fossil state. We are not 

 aware of its general occurrence in other formations than 

 the inferior Oolite, and the Kimmeridge Shale or Oxford 

 Clay; two exceptions may however be mentioned, for 

 casts are found in a Limestone bed in Norway, which is 

 probably contemporaneous with the chalk, but whose 

 characters are much disguised, and some minute species 

 (Scissurellas) are found in the Calcaire grassier. It appears 

 to us indeed probable that it may be also found in other 

 intermediate beds, though we are not aware of the cir- 

 cumstance. 



We have not been able to ascertain whether this 

 Genus should be arranged with the Turbinidai or the 

 Trochidae, for the different species which may properly be 

 called Pleurotomaria differ in form very much, some of 

 them having the exact form of Trochi and others resem- 

 bling Turbines. This can only be ascertained upon the 

 discovery of the operculum, if indeed it possess any, 

 which in the Turbinidse is thick and testaceous, and in the 

 Trochidae thin and horny : though spiral in both. We can 

 only further state with respect to its place in the system, 

 that it appears to us to bear the same relation to Trochus 

 or Turbo, that Fkurotoma does to Fusus or Fasciolaria. 



