14 PROCEEDINGS OF THE NATIONAL MUSEUM vol. los 



MS., a), is not enamel, nor in any way enamel-like, but clearly a 

 special variety of dentinoiis tissue (mesodentine) ; and (b) a lower part 

 of typical aspidin which forms thick walls around a basal, "pulp" 

 cavity (b.as, pea, fig. 3). In the tubercles of early stages of growth, 

 the upper layer is, just as in Astraspis (0rvig, in MS., a), devoid of 

 real dentinal tubes similar to those in dentine proper (e. g., in the 

 tubercles and ridges of the exoskeleton in Eriptychius and post-Ordo- 

 vician Heterostraci in general), but it is, however, pierced throughout 

 its height by numerous very fine canals which correspond, in a broad 

 sense, to delicate ramifications of such tubes. In the tubercles of 

 consecutive stages of growth, on the other hand, this layer is to some 

 extent suggestive of the dentinous tissues in the exoskeleton of various 

 early Osteostraci (0rvig, in MS., a) in that it consists of a thick outer 

 part with the same fine canals as those just referred to (ezul, fig. 3), 

 and, in addition, of a thin, inner part containing a system of short, 

 stemlike dentinal tubes or, in some cases, elongated cell-spaces, from 

 which these fine canals issue in a superficial direction {bzul, dt" , 

 fig. 3). A similar inner part of the dentinous tissue, exhibiting den- 

 tinal tubes or cell-spaces, is absent altogether in all tubercles of the 

 exoskeleton in Astraspis. 



The aspidin in the lower part of the tubercles is perfectly similar 

 histologically to that in the middle and basal layers of the plates and 

 scales. As in Astraspis, it is penetrated by fine fibers of Sharpey 

 radiating from the "pulp" cavity (Bryant, 1936, p. 419, pi. 3; pi. 4, 

 fig. 2; pi. 7, fig. 2; Gross, 1954, p. 80, pi. 3, fig. 2; pi. 5, figs. 3, 4). In 

 the tubercles of early stages of growth this cavity is always undivided 

 and completely separated from the upper dentinous tissue by the thick 

 layer of aspidin which forms its roof (see Gross, 1954, pi. 5, figs. 3, 4). 

 In the tubercles of subsequent stages of growth, however, a system 

 of fau-ly thin, straight vascular canals issue in a radiating manner from 

 the upper part of the "pulp" cavity, terminating close beneath and 

 sometimes almost at the boundary between the aspidin and the outer 

 dentinous tissue {can.pca, fig. 3). This system of vascular canals 

 has no connection whatever with the short dentinal tubes or cell- 

 spaces in the inner part of the dentinous tissue. A similar canal 

 system is not developed in the tubercles in Astraspis, but may possibly 

 have existed in the predecessors of that form (0rvig, in MS., a). 



The nature of the hard tissues of the tubercles will be dealt with 

 elsewhere (0rvig, in MS., a) and will not be commented upon further 

 here. Of interest to us in the present connection is above all the 

 fact that, by their micros tructure, the tubercles of early stages of 

 growth invariably are sharply distinguishable from those of subse- 

 quent stages of growth (clearly shown, e. g., by sections prepared of 



