JOHAN KI^ER. [SEC. ARCT. EXP. FRAM 



Outer Characteristics. The plate is quite flat; length 

 18 mm., breadth 15 mm. and thickness 1.5 mm. Only the upper sur- 

 face is visible; it shines brightly like black enamel, and the sculpture is 

 very beautiful and characteristic. (PI. II, fig. 5; PI. Ill, figs 12). By 

 the naked eye it appears to be finely and regularly tuberculated as is 

 the case with many coccosteomorph fishes, but even a slight enlargement 

 shows that the sculpture is markedly psammosteid. 



The latter is composed of fairly regular rows with markedly ribbed 

 denticles, the points of which do not lie in the centre, but are quite 

 peripheral to the longitudinal direction of the rows. The highly gliste- 

 ning denticles for this reason when slightly magnified have a unique 

 arrow-like form, which however is less striking under higher power. 

 We then observe that the denticles have fine, sharp, and to some extent 

 crenulaled ribs, which all run towards the peripherally situated point. 



The length of the denticles is 0.7 0.9 mm.; the breadth is the same. 

 Now and then we see quite small ones. The spaces between the former 

 are seldom wholly extended; here and there, however, we notice that 

 they run almost directly towards one another. 



Interior Structur e. Microscopic examination shows that the 

 structure is markedly psammosteid. (PI. Ill, fig. 3). The denticles are 

 closely situated, but between them open spaces without skeleton sub- 

 stance are clearly visible. The pulp cavities are very wide and open 

 downwards with wide orifices, communicating in this manner with the 

 underlying vascular canals. The latter are narrow and irregular above, 

 wilh thick walls that as is usually the case show a lamellar structure 

 without bone cells. Deeper down the canals and lacuns become wider 

 and more regular, with thinner walls. Underneath there are again 

 traces of narrower canals which render probable the original existence 

 of a basal layer. The plate, however, is incomplete at this point. 

 Farthest to the right, beyond the part of the microscopical section shown 

 in the illustration, (a transversal section), the somewhat close network 

 grows larger and larger, until at length it occupies the whole thickness 

 of the plate, which at this point becomes thinner. We may therefore 

 presume that at this point there has been a free margin. Between the 

 denticles the canal system opens upwards in pores. 



2. Another quite small fragment (S. I. 81 a) is shown in PI. II fig. 7, 

 5 times enlarged. The denticles are here greatly extended; as however 

 the strong ribs, the peripheral points and the large pulp cavities agree 

 in all respects with those of the above specimen, there can be no doubt 

 that they belong to one and the same form. 



