﻿183 OSTEOSTRACI. 



described, is the series of vertically elongated lateral scutes (PI. X. 

 fig. 1, Xo. 1, and fig. 4, d.l.). All these are more or less upright 

 in position, except towards their superior extremities, where they 

 not only become sharply bent forwards, but are also considerably 

 narrowed. There is no modification immediately behind the shield 

 suggesting the presence of a splint system in connection with a pec- 

 toral arch, and all the plates, to the number of 46, are of about equal 

 width as far as the position of the dorsal fin ; more posteriorly, the 

 lateral scutes are relatively broader and bent forwards at both ex- 

 tremities. Above (as shown especially in PL X. fig. 1, No. 1), the 

 crest of the trunk is formed by a single median series of large scutes, 

 TV-shaped in transverse section, connected and on a level with the 

 *' posterior ridge " of the shield. For almost the whole of the dis- 

 tance between the shield and the dorsal fin, this ridge is very high 

 and acute, the angle between the two lateral halves of the scutes 

 being extremely small ; more posteriorly, the ridge seems to sink, 

 not being angulated, but gently arched from side to side. While 

 the lateral and ventral scutes are distinctly imbricating, many of 

 these ridge-scutes seem to afford very little provision for flexibility, 

 three or four, indeed, being sometimes fused together in front of 

 the dorsal fin ; they are broader than the lateral scutes, each corre- 

 sponding to one and a half or two of these, but more posteriorly 

 their width is exceeded by that of the laterals. The extremity of 

 the tail is unfortunately too imperfect to show the precise characters 

 of its dermal armour. 



No tuberculations are to observed upon the scutes, the external 

 surface, when preserved, apparently only exhibiting the extremely 

 fine, short striae, which are also seen upon the anterior shield be- 

 tween its tubercular ornament. Most of these striae are in the 

 direction of the long axis of the trunk, and, when highly magnified, 

 they have a beaded appearance. The free posterior border of all the 

 scutes is destitute of serrations. 



On comparing the arrangement of the dermal armour thus 

 described with that already made known by Lankester in the " sub- 

 genus Emejplialaspis" it will be found to agree in most essential 

 particulars. Many of the points of difference are very possibly to be 

 accounted for by imperfections in the original fossils, the only 

 marked contrast being found in the ventral scutes, which are directed 

 forwards instead of backwards, as determined in the type species 

 (Lankester, pi. xi. fig. 2). It is also considered probable that in the 

 latter the ventral pair of scutes is divided by sutures into four sym- 

 metrical pairs ; that the upper median scutes are paired, not azygous ; 

 and that immediately behind the dorsal fin arrangements change, 



