Dr. A. S. Woodward on Psammosteus tajdori. G51 



Bpeciinens which have already been referred to Psammosteus 

 on account of their external ornament *. They are flattened 

 probably in a horizontal plane, and, so far as preserved, they 

 are solid, though with comparatively open tissue in their 

 middle layer. At the broad base their tissue has a longitu- 

 dinally fibrous aspect, while the blunt posterior apex (fig. 3), 

 which tends to be falcate in shape, is ornamented on both 

 flat faces by crowded large stellate tubercles^ which are 

 smooth at the summit. The outer border of the apical extre^ 

 niity is smooth, and appearances suggest that this condition 

 is due to wear during life. 



The mode of insertion and attachment of the paired lateral 

 spines is uncertain, but their bases are partly obscured by 

 an obviously flexible mosaic of irregular polygonal tessera?, 

 which are very "thin in comparison with the median dorsal 

 plate. All these tesserjE consist of stellate tubercles fused 

 together, without any added basal layer, and some of those 

 underlying the base of the spine on the right side of the 

 fossil (fig. 5) exhibit the relatively large tubercle near tlic 

 centre, which has already been mentioned by Traquair as 

 characteristic of the ornament of the dorsal plate of P. taylori. 

 Owing to imperfections in the fossil, the extent of this 

 covering of polygonal tesserae on the sides of the fish between 

 the dorsal and ventral plates cannot be determined; but on 

 the left side the region bordering the anterior half of the 

 dorsal plate is invested with large stellate tubercles, witii 

 smooth (perhaps abraded) apices, which appear to have been 

 all separate, and are certainly not fused into groups (fig. 4). 

 These form a layer as thin as the polygonal tessera3, and they 

 are seen to cover the lateral border of the fish, without any 

 marginal plates. 



By a fortunate fracture of the rock the left side of tlie 

 median ventral plate is exposed from within (fig. 2). This 

 plate (y.) is smaller, thinner, and much less convex than the 

 median dorsal, apparently ovoid in shape, and somewhat 

 longer than broad. Its inner face bears some very prominent 

 irregularly arranged pustules (fig. 6). Its outer face is 

 scarcely seen, but one fragment of the antero-lateral portion 

 exhibits flat-topped, large, stellate tubercles, all closely 

 arranged and not in tesserae. 



In the region of the head, just in front of the median dorsal 

 plate, there are also traces of dermal armour, but these are 

 too imperfect for their nature to be determined. A mass of 



* A. S. Woodward, Catal. Fos&. Fiskes Brit. Mus. pt. ii. (1891) 

 p. 126. 



43* 



