STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA. 387 
cular bones, as well as the branchiostegal rays, are extremely alike. The 
hyomandibular is a rod-like bone, without evidence of appended symplectic ; 
the bony palate consists of three elements—pterygoid, mesopterygoid (?), and 
quadrate—similarly related to each other as in the Paleoniscide, and the 
mandible is also identical in its construction. 
7. The scales of Hurynotus are quite conformable in shape to the type 
characteristic of the Paleoniscide, and the dissimilarity observed in the 
other genera of Platysomide is not of so essential a character as has been 
supposed. 
On the other hand, the differences between the two groups, though striking 
enough, are quite insufficient to conceal the close affinity between them. 
1. In the Platysomide the body tends to become deep and short, and to 
assume an ovoid, circular, or rhombic contour. 
2. Though still conformable in essential morphological features to the Pale- 
oniscoid type, the cranial osteology in the Platysomide has undergone a 
remarkable modification characteristic of the family. 
In Palwoniscus, as in the recent Polyodon, the direction of the axis of the 
base of the skull continues forwards that of the vertebral column in pretty 
nearly a straight line. The premaxille are very small, the anterior frontal 
and the median superethmoidal short, and the latter forms a prominence 
over the front of the mouth. The mouth, with its enormously wide gape, is 
itself, as it were, drawn backwards by the great posterior obliquity of the 
hyomandibular, and, coincident with this, the orbit assumes a remarkably 
anterior position close to the snout, and also close above the front part of the 
mouth, the upper margin of the maxilla being consequently suddenly excavated 
or cut away to make room for it. 
In the Platysomide, on the other hand, along with the deepening of the 
body, the line of the base of the skull, assuming a downward and forward slope, 
forms an angle with that of the vertebral axis, the contour of the top of the 
head becoming also more or less steep and inclined. Altering its backward 
obliquity, the hyomandibular has now become, as it were, in pendulum fashion, 
swung forwards, so as to assume either a vertical or a slightly forward as well 
as downward direction, while the bones of the ethmoidal region become 
elongated downwards and forwards. The mouth is thus carried downwards 
and forwards, and becomes less wide than in the Paleeoniscide, and more or 
less “prognathous,” while the nostrils and orbits, remaining behind, appear 
remarkably high up in relation to the mouth and snout, while all that remains 
of the ethmoidal prominence of the Palzeoniscidze is a slight convexity in the 
contour of the head in front of the orbits. The maxilla consequently no longer 
requires to have the front part of its upper margin cut away to accommodate 
the eye, and appears as a simple triangular plate, and a large anterior subor- 
VOL, XXIX, PART I. 5G 
