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BRITISH BELEMNITES. 



DIAGRAM 11. 



Regis, the long advancing flange is clearly traced against the triple conotheca. The 

 septum itself is of unusual construction, as represented in the second figure. It contains 

 three plates — the middle one clear and sparry, and of uniform thickness ; the upper one is 

 quite dark, and thicker in the middle, like a periscopic convex lens ; the lower one is thinner 

 in the middle, like a corresponding concave lens. 



In a specimen of B. tornatilis from the Oxford 

 Clay the conotheca is triple, the two inner laminae being 

 somewhat curved in conformity to the short abutment 

 of the septa (diag. II). The appearances which have 

 been described are not always easy to eliminate from 

 the various shaded and crystallized laminae which 

 overlie the septum, and present delusive appearances 

 of structure. Bisulphides of iron, zinc, and lead, and 

 carbonate of lime in a variety of aspects, form layers 

 on the walls or fill up entirely the chambers. 



According to my observations, there is reason to 

 expect that the phragmocone will afford specific cha- 

 racters more definite, if not so often available as those 

 of the guard. 



When the conotheca is removed, and the cast of 

 the chambers appears, the impressions of the septal 

 flanges remain in several species very plainly on the cast, and cause undulations in the 

 exterior outline of the conotheca. 



Through each transverse plate is a perforation, near the ventral margin, formed by the 

 retroflexion there of the laminae of the plate. These reflected parts of the plate are some- 



DIAGRAM 12. 



DIAGRAM 13. 



times found to be expanded in the interseptal spaces, as happens to many Orthocerata and 

 Nautili. This is represented by Voltz in a specimen of B. Aaknsis} The series of 



1 ' Observ. sur les Belemnites,' pi. i, fig. 3. 



