﻿MYRIACANTHIDJ2. 45 



broad, laterally compressed and pointed, irregularly and widely 

 spaced, occasionally present on part of the longitudinal median lino 

 of the posterior face ; a few of the posterior denticles distally directed 

 downwards, the others pointing upwards. Hinder upper tooth 

 about twice as long as its maximum breadth. Maximum thickness 

 of presymphysial tooth about one third its breadth, and more than 

 twice as great as the thickness of the inner layer of dentine, which 

 is continuous and uniform ; outer face of the tooth gently convex, 

 the inner face slightly concave, but nearly flat. 



The genus Prognaihodus was founded upon the dentition of this 

 species. 



Form. Sf Loc. Lower Lias : Lyme Regis, Dorsetshire. 



(i.) Dorsal Spines. 



P. 6095. One of the type specimens of Myriacanthus paradoocus, 

 figured by Agassiz, torn. cit. pi. vi. figs. 1, 2, and previously 

 figured, without name, by De la Beche, loc. cit. 



Old Collection. 



P. 3067. Another of the type specimens, figured by Agassiz, ibid. 

 pi. vi. fig. 3. Enniskillen Coll. 



P. 3174. A very large crushed spine, about 0*65 in total length. 

 The slender, compressed distal extremity is preserved, 

 destitute of tubercles for a short extent ; and immediately 

 below this space are remains of a few of the characteristic 

 large posterior denticles. At about the middle of the 

 spine, some of the last-mentioned denticles are unbroken, 

 showing their acutely pointed, upwardly curved form. 



Enniskillen Coll. 



P. 3068, P. 3196. The greater portion of two equally large spines, 

 the first being almost uncrushed and displaying several of 

 the denticles. Enniskillen Coll. 



P. 1736. Much crushed similar specimen. Egerton Coll. 



P. 6179. Imperfect large spine, with denticles. Purchased, 1890. 



P. 6221. Fragments of a very large spine, showing part of the 

 smooth distal extremity. 



P. 3071. Distal half of a somewhat smaller spine, with well- 

 preserved denticles. Enniskillen Coll. 



P. 1737. Similar, but more imperfectly preserved specimen. 



Egerton Coll. 



