TEGUMENTARY ORGANS 42 1 



shown that the resemblances between the scales and the teeth of 

 Placoid fish extend even to their mode of development. If the pulp 

 contained in the central cavity of the spine-like scale of a Raia clavata 

 be pulled out,globular calcareous masses of yo^^-ijof an inch and upwards 

 in diameter, and either solitary or adhering together in masses, will 

 be found to be attached to its surface. " These globules are exactly 

 analogous to the dentine globules described by Czermak, which in 

 human teeth afford the formative material for the matrix of the 

 dentine. What, however, appeared to me especially worthy of notice 

 was the circumstance, that the most distinct and beautifully branched 

 canals, having exactly the same appearance as those in the substance 

 of the spine, were already visible in these isolated calcareous bodies, 

 and on carefully examining the fine processes of the canals, no doubt 

 could exist that they were only interspaces or gaps. On carefully 

 adjusting the focus, in fact, it was obvious that one of these large 

 calcareous globules is itself only an agglomeration of many smaller 

 globules, and it could be observed that the gaps left between the 

 latter became the fine processes of the tubules. From these facts, I 

 believe that the correct mode of conceiving the growth of the sub- 

 stance of the spine is, to suppose that the calcareous matter is excreted 

 from the vessels of the pulp, and then in all probability combined 

 with organic matter, runs into smaller masses ; these unite together 

 into larger ones, and become applied to the inner surface of the 

 central cavity, coalescing, and thus adding to the thickness of the 

 spine. Between the calcareous globules, however, canalicular gaps or 

 tubules remain, which form a connected network and communicate 

 with those branched cavities which already exist in the spine. 



The scales of the Sharks and the dermal spines of the Rays, then, 

 (and I would draw particular attention to this result,) are perfectly 

 identical in structure with the teeth, even to the absence of 

 nerves in the pulp, and must be united in the same structural group. 

 I have already (On the Skin of Fresh-water Fishes, Zeitschrift fiir 

 Wiss. Zool. B. iii. H. 4.) pointed out the close affinity between the 

 scales of a number of osseous fishes and their teeth : and scales like- 

 wise present globules of calcareous matter, which become fused 

 together to form the homogeneous substance of the scale. A process, 

 corresponding with that which occurs at the surface of the pulp in the 

 teeth and cutaneous spines, here takes place from the surface of the 

 sac of the scale (Schuppentasche). The scales of osseous fishes, the 

 spines of the Rays, and the scales of the Sharks, therefore, all belong 

 to the series of dental structures, which in no respect interferes with the 

 entrance of true bony tissue (like the " cement " in the higher animals) 



