766 MR. E. S, GOODRICH ON [NoV. 12, 



■a vascular network from which arise branching dentinal tubules. 

 Rohon considers that the tubercles represent denticles fused 

 on to the surface ; they appear to me rather to represent stages 

 in the degeneration of cosmine, than in its formation. The same 

 may be said of Glyptolejns (p. 760) j here the middle vascular layer 

 is to a great extent exposed, forming ridges on the upper face 

 quite similar to those found on the scales of the Rhizodonts. 



The scales of the Rhizodontidse I have been able to examine, 

 such as Rhizodus, Strepsochis, and Eusthenopteron, are rounded, 

 relatively thin, and with a superficial ornamentation of ridges and 

 tubercles (PI. ^LY. fig. 14). The bulk of the scale is made up of 

 the usual parallel bony laminae of isopedine, which merge above 

 into the vascular layei'. The ornamentations have no resemblance 

 to cosmine in structure, and are directly continuous with the bony 

 trabeculae of the vascular layer, being formed of the same sub- 

 stance (PL XLIV. figs. 6 & 8). Where the tubercles ai-e very large, 

 they appear to be formed to a great extent by the upturning of 

 the isopedine lamellse at the growing edge of the scale (fig. 7). In 

 some regions of the scale of Rhizodus this overturning of the laminas 

 is very pronounced ; yet it never results in the deposition of a con- 

 tinuous covering, and only occm^s at the edge. Nevertheless, in 

 JEiisthenopteron and Rhizodits the ornamentation seems to have 

 grown, to some extent, by the addition of new bony layers on the 

 outer surface ; and we may suppose that, the cosmine having dis- 

 appeared, the bone-forming cells of the middle layer emerged on the 

 outer face and continued to secrete there. But when once formed, 

 the middle layer itself does not appear to have grown appreciably 

 in thickness, though new laminfe were always being added to the 

 lower strata. The ridges are often very numerous and regular, 

 with an elaborate network of canals below opening by many 

 apertures on the surface at the bottom of the valleys. This 

 structure is also seen in Glypiolepis ; in which, however, here 

 and there the tubercles have the appearance of cosmine (p. 760)*. 

 Gyrop)tycliius, alone among the Rhizodonts, has preserved a thick 

 shiny scale ; and here can be seen an outer layer of true cosmine 

 as in Osteolepis. 



With regard to the Coelacanthidfe our knowledge is still very 

 incomplete. It has been clearly established by Williamson (26) 

 that some genera such as Macropoma have typical denticles, with 

 dentine cone and pulp-cavity, fused on to the outer surface of 

 their scales and dermal bones (p. 760). His account I can fully 

 confirm. The scale consists of an inner layer of isopedine, 

 an intermediate layer with vascular canals, and lastly of the 

 denticles on that hinder region which is not overlapped by neigh- 

 bouring scales. In Coelacanthus are found the first two layers ; 

 but instead of superficial denticles, there are elongated tubercles 

 or shiny ridges which in section appear merely as hollow a]:'ches 



* We sliall await with tlie greatest interest a description of tlie scale of Tarrasius ; 

 doubtless it will throw much light on the problem of the systematic position of this 

 ntvj imperfectly known but important genus. 



