88 FISH-REMAINS OF THE CARBONIFEROUS ROCKS. 



occupied by soft tissue which decomposed and left spaces 

 which have been filled in with a greyish brown limestone 

 paste. From this spongy basal substance there springs, 

 where the base passes into the crown of the tooth, a series of 

 vascular canals, which radiate upwards towards the outer 

 and upper surface of the tooth, and are filled with a red fer- 

 ruginous material. These vascular canals, the presence of 

 which, too-ether with the dentinal tubules that branch out 

 from them, causes the tissue to be called vaso-dentine, occa- 

 sionally branch and pass to the surface, forming, in this 

 specimen of a well-worn tooth, the punctate pits of the 

 surface. AVe have ground down the apex of the tooth, so as 

 to show a section at right angles to the direction of the vas- 

 cular canals, which appear as red spots on a pinkish white 

 ground. The edges of this section shade off into the outer 

 surface of the worn tooth, and by comparing the two we 

 are able to state that in this specimen the pitting of tlie 

 surface, as it now exists, is the expression of the vascular 

 canals. We draw attention to this point because whereas- 

 Agassiz stated that the punctate crown of Psammodus and 

 similar teeth was related to the internal structure of the 

 tooth, Owen affirmed that the impressions of the surface are 

 entirely unconnected with the vascular canals or internal 

 structure. Sir Richard Owen regarded them as so formed in 

 the tooth germ, and as only natural adaptations to the habits- 

 and food of the species. This difference of opinion between 

 two such careful observers may perhaps be explained on the 

 supposition that Agassiz examined worn teeth while Owen 

 described an unworn specimen. We have not been able to 

 confirm Owen's observation ; but since the specimens we have 

 examined are all more or less worn, this does not show that 

 the observation was incorrect. 



Examined under the microscope, a thin slice of a Psammo- 



