STRUCTURE AND AFFINITIES OF THE PLATYSOMID. 383 
also with a peculiar accessory premandibular piece, the “ Vorkiefer” of the 
Germans, which carries a few “ incisors” like the opposed premaxilla. It need 
hardly be said that nothing at all like this remarkable arrangement occurs in 
any one of the Platysomide, not even in Cheirodus, in which the margin at 
least of the maxilla is edentulous, but whose palatal teeth are borne upon the 
moveable pterygoid bones like those of Eurynotus. 
In many other respects the head in the Pycnodonts differs remarkably from 
that in the Platysomidz. For instance, the greater part of the cheek is 
covered by a mosaic of small polygonal plates, as is also the throat between 
the mandible and the lower extremities of the clavicles. Different opinions 
have been expressed as to the number of opercular pieces ; like QuENsTEDT, I 
_ have myself been only able to distinguish one, the operculum, below which are 
only two narrow branchiostegal rays instead of the long series, which in the 
Platysomide extends forwards below the mandible; and from this, as well as 
the appearance of the throat, it seems pretty clear that the external branchial 
cleft or opening was limited below, as in the modern Plectognathi, and did not 
form the long slit seen both in the Platysomide and Paleoniscide. 
2. The clavicle differs in shape at its lower extremity from that of the 
Platysomide, and the infraclavicwar plates are altogether absent. 
3. The vertebral axis is notochordal, but the neural and hemal spines 
spring from well-developed hemivertebre, which in Pycnodus join each other 
by suture above and below on the sides of the chorda. The neural and hemal 
arches are connected with each other by horizontal denticulated articular 
processes, and the spinous processes are very long, the neural ones in front of 
the dorsal fin reaching nearly to the margin of the body. The abdominal 
region is provided with long and well-developed ribs. 
4. The dorsal and anal fins are supported each by only one set of inter- 
spinous bones, and these have their proximal extremities inserted between the 
extremities of the neural and hzmal spines, as in modern fishes. 
5. Fulcra are entirely absent from all the fins, and the rays of the dorsal 
and anal correspond in number to their supporting interspinous bones, to whose 
extremities they are articulated. 
6. The caudal fin is only semiheterocercal, and in those genera in which 
the entire body is covered with scales (Gyrodus, Mesturus), the heterocercy 
is almost completely masked, when the scales are well preserved in the caudal 
region. 
Many genera, mostly founded upon fragmentary remains, have been added 
to the Pycnodontidz, chiefly or only on account of the possession of flattened 
teeth, and in some of these instances, ¢.g., the reptilian Placodus, the reference 
to this family has been subsequently found to be rather wide of the mark. But, 
looking at these forms whose claim to be considered as Pycnodonts is established 
VOL. XXIX. PART I. 5 F 
