Lamarck's Genera of Shells. 79 



progressively increasing in size. Spire excentric, sublateral. Septa 

 imperforate. 



Most of the cristellariae, are flattened, crest-shaped shells, their 

 septa are visible externally ; the chambers are elongated, subra- 

 diated, of the whole breadth of the whorl which contains them, 

 and have an excentric, almost lateral axis. 



Type. Cristellaria squammula *. 



No further description. PI. vi. Fig. 218. 9 Species, all recent. 

 3. Orbiculina f. 



Shell subdiscoidal, multilocular : whorls contiguous and com- 

 pound ; spire excentric ; chambers short, very numerous ; septa 

 imperforate. 



The chambers of the orbiculina seem to be of two kinds, they 

 traverse each other, and render the whorls, as it were, compound. 

 Most of the species of this genus are flattened, or compressed. 

 The aperture is narrow, in the form of an arched, transverse fissure, 

 and appears common to the chambers of the last row. 



Type. Orbiculina numismalisl. 



No further description. PI. vi. Fig. 219. 3 Species, all recent. 

 [To be concluded.] 



Art. VII. On an Arenaceo-calcareous Substance found near 

 Delvine in Perthshire. By J. Mac Culloch, M.D,. F.R.S. 



The present notice relates to a substance hitherto undescribed ; 

 still limited, as far as I know, to the spot named in the title, and 

 possessing some resemblance to an object well known to minera- 

 logists, the arenaceo-calcareous spar of Fontainebleau. 



The present course of the Tay through the plain of Stormount 

 is accompanied by high terraces of rolled stones, gravel, and sand, 

 the ruins of the mountains from which it traces its many-headed 

 origin, and the remains of an alluvial plain, through which it is 

 still deepening its way, leaving these deserted records of its cor- 

 rosive power. 



* From sguuma, a scale. t From orlieulus, a Utile orl>. 



% From numisma, a piece qf money, 



