STRUCTURE OF BONE IN PISHES. 



81 



in the scales of Lejndosteus, and by myself in those of various 

 extinct Amioids and Lepidosteoids (1, 2). They branch, as a 

 rule, only at their inner end, and pass outwards to the surface. 

 In the living tissue they are occupied by the long protoplasmic 

 processes of large cells on the surface of the scale. Probably, 

 these remarkable cells are merely modified bone-cells, which, 

 instead of becoming buried in the ostein matrix, remain outside 

 it while retaining their connection, by means of the long process, 

 with the place they originally held. This interpretation is 

 illustrated in text-fig. 13. 



Text-fiff. 13. 



Diagram illustrating the structure and growth of lepidosteoid bone. 

 h.l., bony lamella ; l.t., lepidosteoid cell ; o., osteoblast or bone-cell. 



It follows that the Actinopterygii can be classified into two 

 large groups according to the structure of their scales : the first 

 is distinguished by the possession of palyeoniscoid scales, and 

 contains the Chondrostei (with which the Polypterini should 

 probably be placed, 2) ; the second group contains the Amioidei 

 (Protospondyli, Pholidophoridse, and Oligopleuridae) and the 

 Lepidosteoidei (^theospondyli). 



Now it might be expected that this striking difference in the 

 histological structure of the true ganoid scales would also be 

 found in the cranial plates and other dermal bones of these fishes, 

 and this is indeed the case. The dermal bones resemble the 

 scales not only in appearance, but also in microscopic structure. 

 Often the resemblance is so close that they cannot be 

 distinguished ; but the dermal bones may, of course, lose the 

 covering of ganoine, as sometimes happens with the scales them- 

 selves in the more modified forms. Thus, whereas lepidosteoid 

 tubules are never found in any part of the skeleton of the 

 Polypterini or Chondrostei, they occur in the dermal bones of all 

 the recent and extinct Amioidei and Lepidosteoidei I have been 

 able to examine, with a single possible exception {Oligo2)leurus) 

 to be discussed later*. 



* I am much indebted to Dr. A. Smith Woodward for the supply of most of the 

 material on which these researches were carried out, and to Miss R. Harrison for 

 the preparation of a large number of microscopic slides of the bone of various fishes. 



Proc. Zool. Soc— 1913, No. VI. 6 



