STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA. 353 
plate, which in the Palzeoniscide I have called “ interoperculum,” for reasons 
given in my memoir on the structure of that family, which interpretation I 
therefore retain for it in the Platysomide. The anterior margins of the 
operculum and interoperculum form one continuous line, nearly vertical in 
direction, following that of the hyomandibular, and slightly concave in contour, 
not angulated at the junction of the two plates, as in the Palzoniscide. Now, 
fitting on to this line in front, and covering the hyomandibular and a portion of 
the cheek, is a narrow vertical plate (p. op.), broadest in the middle and pointed 
below, and filling in the space between the operculum and interoperculum 
behind, and the posterior margin of the maxilla and the suborbital chain in 
front. It is equally evident that this plate is the homologue of that which 
Ihave marked “preoperculum” in the Paleoniscide, though much modified 
in form by the altered direction and configuration of the hyomandibular. 
Succeeding the interoperculum and extending between the rami of the - 
mandible, there is on each side a series of branchiostegal rays (br) in the form 
of narrow, slightly curved, imbricating plates, whose exact number I have 
not in any case been able to ascertain. In front, however, exactly the 
same arrangement is found in Hurynotus as that which prevails in most 
Paleoniscide (¢.9., Palewoniscus, Elonichthys)—viz., the anterior plate of each 
lateral series is much broader than the rest, and there is a median lozenge- 
shaped one behind the symphysis corresponding to the median “ jugular ” in 
Amia, and in many extinct Lepidosteid forms (Eugnathus, Dapedius, &c.). 
The exposed surfaces of the bones of the shoulder, and of the exterior of 
the head of Hurynotus are brilliantly ganoid and ornately sculptured, usually 
with tolerably coarse and prominent corrugations and furrows. 
Owing to the great density of the scaly covering, no internal skeleton can 
be seen in ordinary entire specimens of Hurynotus, though numerous scattered 
ossicles, apparently spinous processes and interspinous bones, may be observed 
lying among the scales in disjointed examples. But the Belgian specimen of 
EBurynotus figured by DE Kontncx (op. cit. pl. iii. fig. 1a), which I have had the 
privilege of examining, displays in the dorsal region, where the scales of the 
left side have been removed with the counterpart, a considerable portion of the 
internal skeleton im situ, compressed against the inner surfaces of the scales of 
the other side of the body. What is here seen of the internal skeleton consists 
of a set of short neural spines, surmounted, as in the Paleeoniscide, by two sets 
of interspinous bones, proximal and distal, those of the distal set being also less 
numerous than the dorsal fin-rays which they support. As in Platysomus, the 
proximal set of interspinous bones may be observed extending in front of the 
dorsal fin towards the occiput. These little bones seem to have been over- 
looked by Professor Dz Koninck both in his description and figure, but, 
indeed, very close examination of the specimen is necessary for their detection. 
