382 RAMSAY H. TRAQUAIR ON THE 
the usual conceptions of the limits of such a zoological division. To my mind, 
those differences express a separation of still wider extent than one of mere 
“ family ” importance. 
Relationship of the Platysomide to the Pycnodontide. 
The Pycnodonts form a remarkable and most distinctly characterised family 
of extinct fishes, which range from the Liassic to the Eocene rocks inclusive, 
and whose zoological position is even yet rather problematic in its nature. 
And the Platysomide seem to be still less related to them than even to the 
Dapediidee. 
The Pycnodonts may be said to resemble the Platysomide in the following 
few points :-— 
1. The shape of the body is deep; the dorsal fin extends from near the 
middle of the back to the tail pedicle, and the anal agrees with that of Platy- 
somus at least, in being nearly the counterpart of the dorsal in form and position. 
The contour of the top of the head slopes steeply downwards and forwards, and 
usually shows a slight convexity in front of the orbit, which is placed rather 
high up and far back ; the snout is pointed and “ prognathic ” in aspect. 
2. The scales, sometimes limited to the anterior part of the body, are mostly 
high and narrow, and have their internal rib or keel coincident with the anterior 
margin, and passing up into a strong spine, which articulates with the bevelled- 
off lower extremity of the rib of the scale next above. 
3. The notochord is persistent. 
The differences, on the other hand, are of a very much more important 
character— 
1. The osteology of the head of the Pycnodontidz is not yet in every 
particular satisfactorily elucidated; certain facts are,however, well established, 
which are completely at variance with anything observable in the Platysomide. 
The most salient feature in the Pycnodont head is the altogether peculiar 
construction of the masticatory apparatus. The maxilla is a thin edentulous — 
lamina, which is rarely seen. The long premaxillary bones carry a few styliform 
or chisel-shaped “ incisor ” teeth, behind which the roof of the mouth is occu- 
pied by a long median bone, supposed to be the vomer, or united parasphenoid 
and vomer. The straight sides of this bone slightly converge anteriorly, so 
that it is narrower in front than behind, it is gently convex longitudinally, and 
bears several rows of rounded or oval flattened crushing teeth, there being usually 
five of these rows, one median and two lateral. This bone, with its formidable 
armature, bites below into a longitudinal hollow formed by the apposed right 
and left rami of the very stout mandible, each ramus being provided also with 
several rows of flattened teeth, usually four, and at its symphyseal extremity, 
