lStiS-190-2. N„.33.] UPPER DEVONIAN FISH REMAINS. 25 



3. Another little fragment (S. I. 81 c), in many respects stands 

 between the two above specimens. The denticles in this instance are 

 more compresed, but they agree in other respects. To some extent 

 they show a maiked alternating arrangement, m a manner similar to 

 that described for a piece of Ps. complimtus (fig. in text 5). 



4. A particularly interesting fragment (S. I. 20) will be seen in 

 PI. V, figs 4—5. It is a piece of a compressed, spine shaped plate with 

 a long oval cross section; all the sides are covered with a very charac- 

 teristic layer of denticles, which are extremely well preserved on the 

 upper side. The upper and lower edges are fracture surfaces. Greatest 

 length 19 mm.; breath below 12 mm.; above 9 mm., thickness below 

 4 mm., above 3 mm. The layer of denticles consists ot elongated and 

 flat denticles. Its breadth is from 0.7 mm. to 0,8 mm., length from 

 1 mm. to 3.5 mm. On the sui'face that is visible they are arranged in 

 angular rows, the [loint of which turns towards the broader basal end of 

 the spine shaped plate. The separate denticles ha\e a markedly fringed 

 edge against this margin, whilst towards the other they rise gently to a 

 smooth and often undulating edge. On greater enlargement, however, 

 this is found to be faintly crenelated. Towards the sides the denticles 

 become smaller. 



I consider these denticles to be of the same type as those of the 

 fragments previously described. They have only been gi'eatly com- 

 pressed and elongated. 



It is worthy of emphasis that the spine shaped element referred to 

 above is not bilaterally symmetrical in construction. One surface, which 

 may be seen in the figure, is indeed slightly concav, whilst the opposite 

 one is slightly convex. The left edge is also slightly concave, the right 

 slightly convex. 



The layer of denticles has fallen off on the lower part of the specimen 

 The underlying vascular canal system therefore becomes exposed; the 

 longitudinal direction follows the longitudinal axis of this element. 



'& 



5. PI. II, fig. 6, shows a small arched plate, (S- I. 69 b) that pre- 

 sumably mav also be refei-red to the above form. A number of strongly 

 crenelated denticles are seen here grouped around a comparatively large 

 central denticle. The microscopic structure of this plate is shown in. 

 PI. Ill, fig. 4. ll agrees well with that described in the case of the 

 specimen first mentioned. 



My material contains several similar small plates. 



Observations. The fragments here described as a new species, 

 Psammosteus arcticiis, are distinctly differentiated both as regards 



