Dr. Mantell on the Structure of the Belemnite. 



17 



species is prolonged into a cylindrical rostrum or beak, which 

 terminates in a conical apex. ]?;„ 2. 



Diagram of the known parts of the structure of Belem- 

 nites Puzosianus. 



a, a, the dorsal basilar processes of the phragmocone. 



b, b, upward extension of the attenuated osselet. 



c, siphunculus. 



d, phragmocone : the transverse lines indicate the septa. 



e, the capsule or outer investment of the guard. 

 /, the distal part of the phragmocone. 



g, the alveolus or cavitv in the guard. 



h, vertical section of the guard. 



i, the solid part of the rostrum. 



k, a siUcus or groove on the ventral aspect of the 

 guard. 



/, shows the continuation of the capsule, in section, 

 continued from e. 



m, diverging parallel stria; observable between the dor- 

 sal processes of the phragmocone. 



n, transverse section of half the diameter of the ros- 

 trum, to show its radiated structure. 



m 



I 



As the solid part of the osselet is generally separated from the 

 upper portion a short space above the apex of the phragmocone, 

 in consequence of the thinness of its Myalls, the Belemnite is 

 commonly found with a conical cavity in the upper part : this 

 hollow w^as termed the alveolus, and the solid part the rostrum 

 or guard ; and \mtil shown by the specimen figured in my first 

 memoir on the Belemnites, no one suspected that the osselet 

 was continued upwards, and formed a thin envelope around the 

 basilar termination of the phragmocone*. 



The osselet of the Belemnite, as is well known, has a radiated 

 structure : it is formed of thin concentric laminse of very minute 

 prismatic trihedral fibres, which are arranged at right angles to 

 the planes of the successive layers : — see the sections, both longi- 

 tudinal and transverse, in fig. 2. The solid part, or rostrum, is 



* The depression observable in the specimen fig. 1, midway between 

 the letters b and c, indicates the fracture of the walls of the osselet, and the 

 point where the Belemnite is usually separated from the other parts. It was 

 by removing large blocks of clay, with the imbedded Belemnites undisturbed, 

 that the instructive examples here figured were obtained. 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. x. 2 



k. 



