﻿STRUCTURE. 



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parallel to the edges of the septa. Measured round the cone, the dorsal region 

 occupies about ' t h , each lateral (or hyperbolic region, as Voltz calls it) about 5th, 

 and the ventral region about £ the circumference. The straight or nearly straight 

 boundaries of the dorsal space are called asymptotes. There is occasionally a straight line 

 along the middle of the dorsal region crossing the " ogivial" dorsal axis ; this line is in some 

 cases elevated, in others depressed. Fine longitudinal striae appear on the outermost layer 

 of the phragmocone especially in the ventral region. 



The proportions of the phragmocone, its transverse septa, chambers, and siphuncle, 

 vary in different species of Belemnites. The axis of the general figure is for the most 

 part a little curved, always in such a manner that the apex is slightly bowed towards the 

 lower or ventral side. If a longitudinal section be made right through the siphuncle, 

 from the dorsal to the ventral line, and the angle of inclination to each other of the sides 

 of the phragmocone be measured, it will be found that this angle grows more and more 

 open as we approach the apex. If another longitudinal section be taken across the cone 

 from side to side the boundaries will be nearly straight, and the angle of inclination of 

 the sides nearly constant. It is this angle which should be given as a part of the 

 specific character. Taking an example from a phragmocone of any Belemnite, the septa 

 are found to be nearer and nearer to one another as the cone grows smaller and we approach 

 the apex. In some Belemnites the septa are circular ; in others elliptical, the longer axis 

 being between the dorsal and the ventral face, and passing through the siphuncle. In 

 B. paxillosm the diameters are as 100 to 108. The curvature of the septal plate is nearly 

 spherical or spheroidal; the arc included is about 50°, but toward the apex greater, even 

 exceeding 90°. The radius grows shorter and shorter in a greater proportion than the 

 diameter lessens. The intervals between the septa are about 5th of the diameter. In 

 a series of septa exposed by section of the phragmocone of B. canaliculatus, the intervals 

 were thus found nearly — 



17, 16, 15, 14, 13, 12, 11, 10, 10, 9, 8; 

 and in another less regular — 



15, 14, 13, 10, 8, 10, 9, 9, 8. 

 In neither case could the septa be counted to the end, but the whole number was estimated 

 to be about thirty in a length of fths of an inch. In a specimen from Lyme Regis ten 

 septa occur in the space of ,^th of an inch near the apex. 



M. Voltz figures a fragment of Belemnites Aalensis containing ten septa in a space of 

 1-83 inch. The angle of this cone being 21°, and its base 114 inch, it is practicable 

 to calculate the probable number of septa, till near the apex they would become untrace- 

 able. The cone would be 3| inches long, and would contain fifty septa before the 

 smallest was reduced to ^th of an inch, and sixty before the diameter was reduced 

 to ith. 



In a pyritous specimen of B. vulgaris, Y. and B., from the Upper Lias of Whitby I 

 count sixty septa, of which the anterior twenty are singularly pressed inwards close up to 



