LEP1D0GAN0IDEI 



133 



Fig. 50 a. 



Tooth of Dendrodus biporcatus 



(nat. size). 



logical series of the British Museum. It is chiefly remarkable 

 for the size and bold sculpturing of the ganoid scales (fig. 50). 

 Large fossil teeth, with the more complex "dendritic" 

 disposition of the tissues, charac- 

 terize a genus (Dendrodus), most pro- 

 bably of the Holoptychian family. 

 The complexity is produced by 

 numerous fissures radiating from a 

 central mass of vasodentine, which 

 more or less fills up the pulp-cavity 

 of the seemingly simple conical teeth 

 of this genus. Fig. 50 a is one of these 

 fossil teeth of the natural size — a, a 

 transverse section ; and fig. 50 h, a 

 reduced view of a portion of the 

 same section enlarged twenty diameters. Thus magnified, a 

 central pulp-cavity of relatively small size, and of an irregular 

 lobulated form, is discerned, a portion of which is shown at p ; 

 this is immediately surrounded by transverse sections of large 

 cylindrical vascular or pulp canals of different sizes ; and 

 beyond these there are smaller and more numerous medullary 

 canals, which are processes of the central pulp-cavity. In the 

 transverse section these processes are seen to be connected 

 together by a net-work of smaller vascular canals belonging to 

 a coarse osseous texture, into which the pulp has been con- 

 verted, and this structure occupies the middle half of the sec- 

 tion. All the vascular canals were filled up by the opaque 

 matrix. From the circumference of the central net- work 

 straight pulp-fissures radiate at pretty regular intervals to the 

 periphery of the tooth ; most of these fissures divide once, 

 rarely twice, in their course — the division taking place some- 

 times at their origin, in others at different distances from their 

 terminations, and the branches diverge slightly as they pro- 

 ceed. Each of the above pulp-canals or fissures is continued 



