STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA. 357 
small and acutely lozenge-shaped, and arranged in bands directed very 
obliquely downwards and forwards, this arrangement commencing as usual 
opposite the lower caudal lobe ; the upper margin of the same part is set 
with large and imbricating V scales. 
In general form the head of Mesolepis (Plate IV. figs. 1 and 9) resembles 
very closely that of Hurynotus. The sloping contour of the top of the head. 
shows the same rounded prominence over the orbit, below which it slopes stil] 
more sharply into the short pointed snout. The bones of the cranial roof are 
delicately sculptured with ridges and granules ; the lines of demarcation between 
at least some of them may be observed in a specimen belonging to Mr Warp. 
A pair of well-marked elongated /rontals (/) occupy the top of the buckler, but 
the boundaries of the plates in the parietal, squamosal, and post-frontal regions 
are obscured by crushing. The rounded prominence above and in front of 
the orbit is formed by a narrow median superethmoidal plate (e), with a broader 
anterior frontal (a. f) on each side; the posterior extremity of the former of 
these is pointed, and received into a notch between the anterior extremities 
of the frontals, while each of its lateral margins show a rounded notch com- 
pleted into a small nasal opening (”) by the inner margin of the adjacent anterior 
frontal. The portion of bone forming the snout in front of the last described 
plates is probably premazillary in its nature. 
The line of the suspensorium passes downwards with a slight forward 
inclination, the operculum (op) is short, the interoperculum (2. op) larger and 
higher ; the preoperculum is not exhibited in any specimen I have seen, but a 
well-developed series of narrow imbricating branchiostegal rays or plates (b7) 
follows the lower margin of the interoperculum. The mazilla (mz) resembles 
that of Wardichthys more than that of Hurynotus ; it is broad behind, pointed 
in front, the upper margin, sloping downwards and forwards, showing imme- 
diately behind the anteriorly directed apex a small rounded expansion; the 
lower or oral margin is gently convex, and displays no teeth when seen from 
the outside, nevertheless, on the internal aspect of this margin there are very 
evident traces of at least tooth-like tubercles. The mandible is short and stout, 
and peculiarly pointed in front ; the anterior part of the upper margin of the 
dentary element is separated off from the rest of the bone by a wide smooth 
shallow groove, below and behind which the surface is ganoid and sculptured. 
On the margin thus marked off is seen a set of peculiarly shaped teeth, all of 
the same size, and apparently in a single row. Each of these teeth, ordinarily 
and as seen in Plate IV. figs. 6 and 7, consists of a head with bluntly pointed 
apex, rounded below, and supported by a constricted neck, which again expands 
so as to form a conical base; but in M. micropterus (fig. 8), they are, as I 
have already stated, cylindro-conical, the apical dilatation and neck-constric- 
tion being scarcely if at all marked. Iam unable to give any account of the 
