172 DENDRODUS. 



the central or corn-stone division of the Old Red Sandstone. The three 

 specimens here described are from Scat- crag, near Elgin. The largest of 

 these is represented of the natural size at PL 62 a, fig. 1. It is conical, 

 subcompressed, subincurved, with a round obtuse summit (a), and a 

 sub-circular base, the margin of which, in the specimen, was 

 rounded off or bent in, and the surface of the base was shghtly 

 depressed in the centre, and rough as if it had been detached from an 

 raichylosed union with a shallow alveolar depression. On each side 

 of the tooth, a little nearer the convex than the concave surface, 

 there is a well marked longitudinal ridge, which gradually subsides a 

 little below the summit. The position of these two ridges, and the 

 degree of compression of the tooth at its middle part, are shown in 

 the outline of the transverse section of the tooth by the side of the 

 figure. The smooth surface of the tooth is traversed by very fine 

 longitudinal linear impressions extending from the base to within 

 one fourth of the rounded apex. 



A transverse section taken one line above the base of this tooth, 

 exhibited a dark irregularly dotted central part, and a lighter peri- 

 pheral structure : it is represented at PI. 62 b, fig. 1. A portion of 

 the same section corresponding with that intercepted by the two lines 

 in fig. 1, as viewed by transmitted light with a magnifying power of 

 fifty linear dimensions, is represented on a reduced scale at fig. 2. 



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 a ; this is immediately surrounded by the transverse sections 

 of large cylindrical medullary 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 medullary canals belonging to a coarse osseous 

 texture into which the pulp has been converted, and this structure 

 occupies the middle half of the section. All the medullary canals were 

 filled by the opaque matrix. From the circumference of the central 

 net- work, straight medullary canals radiate at pretty regular intervals 

 to the periphery of the tooth ; most of these canals divide once, rarely 

 twice, in their course ; the division taking place sometimes at their 



