2S 



.lOIlAN KLEK. 



SEC. ARCT. EXP. FRAM 



ill the skeleton mass. The pulp cavities are generally narrow, canal- 

 like, anil are in connection with the luiiierlying vascular canals hy means 

 of a narrow opening of the natui-e of a canal. The denticles are com- 

 posed, as usual, of fine dentine lulniles; in the spaces between the 

 denticles we also find intermediary hundles of dentine tubules, that 

 emerge from the underlying canal system and rise amongst the surface 

 denticles. Tbis peculiarity is especially characteristic. In the peripheral 



Fig. ."p. Sketi-li stioiving ttie stnichiri' of llif dermal stielelon o( Psammosfeus complica- 

 <(f.5 ufw. sp. Tlie (lerinal skeleton is iiil throiigli in several direction.s. At the top we 

 SIM- llic ili'iiticles with llieir pnlp ravities ami line ilenline tiiljnies: lielween tliPiii 

 llie iiiterinrdiary bundles of dentine Itiliides, and under, the vascalar eanal system 



part iif tbe fragment the v,-t.-,cular canals are narrow with thick walls: 

 faitber inwards thev become widL-r, but tbere too tbe walls are thick 

 with slrongh' marked dark median layer and lamellar structure, fn the 

 longitudinal section we see straight canals, wliich are .sometimes more 

 irregular, and extend in lacunal fasliion, often loitering into connection 

 willi each oilier by ine;nis of large or small (jrifices and fine canals. 



The fragment described in f. shows a similar microscoi)ic structure; 

 Ihe vascular canal system, however, is wider in this case, and in the lower 

 part of the plate the latter has lhiiiiie[' walls. 



