6 Mr. A. S. Woodward on the 



u])tiirned edge of the lateral plates. The median ventral 

 phitc (w.-y.) is remarkably large, almost equilateral, and 

 about as broad as long, and each vcntro-lateral ]jlate is 

 distinctly shown to be continued into an upturned lamina at 

 its outer border. The posterior ventre- lateral plate is com- 

 paratively long and narrow, with a broad transverse thickening 

 on its visceral aspect, corresponding to the ridge that seems 

 to mark the hinder boundary of the abdominal cavity in the 

 Astcrolepida?. The anterior ventro-lateral plate is broader in 

 proportion to its length, but the ])recise form cannot be ascer- 

 tained on account ol' the loss of the front margin. 



The two shields thus described correspond so closely in 

 size that they might have formed the dorsal and ventral 

 armour of one and the same individual. That they pertain 

 to two distinct specimens, however, seems to be proved by 

 the circumstance that the appendage preserved in each case is 

 on the right side, while its superior surface is more or less 

 intact in both. The appendage is hollow and thin-walled, at 

 least at its base, as indicated by the transverse sections (PI. I. 

 figs. 3 a, Z»), and the broad basal portion is directly continuous 

 both with the dorsal armour of the trunk above and with the 

 anterior ventro-lateral plate below, there being no interposed 

 suture or movable joint. The dorsal and ventral faces of the 

 ajipendage are evidently flattened and even, while the lateral 

 borders are sharply rounded ; and where the surface or its 

 impression is distinctly preserved, close parallel series of small 

 tubcrculations are shown to be arranged longitudinally. The 

 greater ]5ortion of the spine-like ])late consists of fibrous tissue, 

 of which the fibres are longitudinal in direction ; but a fortu- 

 nate plane of fracture in the apijcndage attached to the dorsal 

 shield exhibits a sharp line of demarcation between the inner 

 border of the proximal half of tliis element (.s) and a broad, 

 triangular, basal area (h) in which the structural fibres radiate 

 outwards. It is tiius evident that the arrangement agrees 

 })veciscly with that already noted by Newberry in sonie of the 

 type specimens of Acunlhasina from the Corniferous Lime- 

 stone, where the sutine now indicated by minute structural 

 characters is sometiiues open, though quite as often closet!. 



Form, and Loc. lied Micaceous Sandstone, Dickson Bay. 



Acantlmsjiis mi nor ^ sp. n. (PI. II. figs. 2-5.) 



A comparatively small sjieeies. Appendages slender, 

 gently are\u\ted, ornamented with few, conspicuous, rounded 

 longitudinal ribs, which are nodose at distant intervals; 

 prominent lateral denticles at least on the concave border. 



