1898-1902. No. 38.] ll'I'KI! DKVOMAN KISII I5KMAINS. 



2. In the skeleton structure the layer of denticles is most charac- 

 teristic. The denticles have slightly developed pulp cavities which 

 communicate below with the vascular canals by the aid of narrow 

 orifices. Between these denticles we find intermediary bundles of dentine 

 tubules, which emerge from branches of the underlying canal system and 

 rise amongst the surface denticles. 



Description. Some of the best preserved and most characteristic 

 fragments will now be described. 



1. A small fragment, (S. I. 21) will be seen in PI. V, fig. 3. The 

 thickness is considerable, about 3 mm., but it has been even greater, 

 the lower part of the skeleton substance being wanting. The denticles 

 are round, the diameter being 1 1.3 mm. They are considerably arched 

 and have a broad, rounded, central point and strong ribs. 



. Another closely related fragment has on the whole smaller denticles, 

 these having a diameter of not more than 1 mm. In other respects it 

 agrees with the preceding. 



2. A larger fragment (S. I. 13) is represented in PI. V, figs 12. 

 The length of this specimen is 39 mm., breadth 28 mm., and the maximum 

 thickness 7.7 mm. One side, which is depicted here, is somewhat arched, 

 whilst the opqosite one is almost flat. They unite on one side (the left 

 in our figure), in a free, rounded margin, which also is covered by 

 denticles. The latter have partly fallen off, but are very well shown on 

 the left side of fig. 1, and are also shown greatly enlarged in fig. 2. 



The denticles are much smaller than in the preceding specimen. 

 They are closely situated, and oblong rhomboid or oval in form; as a 

 rule ihey are more or less extended, with the longitudinal axis parallel 

 to the free margin. They are slightly arched, without a marked point, 

 and with faintly developed ribs. They often appear arranged in distinctly 

 alternating oblique rows. On the free margin itself they are no longer 

 extended but are farther in. The breadth of the denticles is 0.4 0.5 mm., 

 length about 1 mm. the latter increasing now and then in the inner 

 portion to 1.3 1.4 mm, 



Several fragments of the same character as that described under 2 

 are to be found in the material. 



I n n e r S t r u c t u r e. By the aid of series of microscopic sections 

 (PI. VI, figs 13) I have been able to investigate most minutely the 

 fragments described in 2. Fig. 5 (in the text) gives the structure more 

 diagramatically. 



The denticles, which are slightly arched, will be seen to sland quite 

 closely without intervening spaces; they appear to be almost embedded 



