8 Dr. R. H. Traquair on the Genus Dipterus. 



considered by Prof. Huxley to represent the dentary elements-^ 

 are small, seen principally on the lower aspect of the jaw, and 

 separated from each other by a median suture, we have in 

 Diptei-us a single bone (tZ), on whose external smooth and 

 ganoid surface no median suture can be seen, and which, ex- 

 tending upwards and forwards, forms a rounded enamelled 

 lower lip, whose contour, as seen from above (fig. 2), consti- 

 tutes an arc of more than half a circle. Seen from below 

 (fig. 3) this median "dentary" shows posteriorly a free 

 concave border, continuing the lower margins of the right and 

 left rami uninterruptedly round into each other ; in front it 

 passes round into the labial margin already mentioned. The 

 oral aspect of the bone, just within the lip and above the united 

 S])lenials, is, in one specimen, seen to possess a narrow band 

 of small enamelled denticles, resembling in form those on the 

 ridges of the palatal and splenial dental plates, but irregularly 

 arranged; in fig. 2 these are concealed by the matrix. Now 

 if we look at the jaw from the side (fig. 4) — and the same 

 ap])earance is also to some extent visible from below (fig. 3) — 

 it will be seen that below the posterior half of the labial 

 margin the bone is suddenly and deeply excavated, so as to 

 form on each side a well-marked hollow (?/), bounded below by a 

 sharp margin, which, curving sharply round in front, passes 

 then into the posterior part of the lower lip, overhanging and 

 bounding the hollow from above. Posteriorly the floor of this 

 hollow, which is non-ganoid and crossed obliquely by the 

 suture separating the dentary from the angular element, 

 passes uninterruptedly backwards onto the outer surface of 

 the latter. These hollows are important, inasmuch as they 

 afibrd us an explanation of the real nature of the so-called 

 nasal apertures of Pala^daylms insignis (figs. 5, 6, 7), as we 

 shall see in considering that remarkable fossil further on. 



The large lozenge-shaped parasphenoid of Dipterus (fig. 1, 

 PI. III. jya.sph) is well known ; and its relations to the palato- 

 pterygoid plates are precisely as in Ceratodus • there is, 

 however, no evidence of its having been prolonged backwards 

 as a narrow process along the under surface of the vertebral 

 axis for a little distance behind the skull as in the latter 

 genus. The " palatal dart-head," figured by Hugh Miller 

 (' Footprints of the Creator') is the anterior part of the para- 

 sphenoid broken off. 



The upper aspect of the skull differs remarkably from that 

 of Ceratodus in being covered by a buckler of thick polygonal 

 ganoid plates, whose arrangement has been well delineated by 

 Hugh Miller and by Pander, though it is difficult to trace any 

 exact correspondence between them and the cranial roof-bones 



