OP THE SKULL IK LEPIDOSTEUS OSSEUS. 
477 
Sixth Stage.—Young Lepidosteus, 4 inches 5 lines long. 
In this stage the dermal scutes are so well developed that they can be named and 
classified ; I shall describe them first, and the endocranium afterwards. 
Notwithstanding the size of this specimen it had still the remains of the embryonic 
suctorial disk at the end of the snout (Plate 37, figs. 1—3), forming a pad on the end of 
the premaxillaries (px.), and lying in a horizontal plane. The lower jaw just reaches 
this part, the disk, itself, overlapping it. 
The rostral region of the head is twice as long as the cranial; the opercular bones 
(op., s.op.), pass behind the projecting basioccipital (b.o.) 
The mandibles are two-thirds the length of the head, and as in embryo Progs, are 
articulated to the quadrate in front of the eye-hall {e.) ; in old Frogs the condyle 
of the quadrate may reach as far backwards as the opercular folds do in this Fish. 
The bony scutes of the hinder part of the head and face do not differ much from 
those covering the body, except in size ; but in the rostral region, both above and 
below, but especially below, many of the bony plates are styles of great length and 
tenuity; this is a specialization quite like that which is seen in the skulls of 
longirostral Birds, and in some extinct Sauropsida, e.g., the Ichthyosaurus. 
Most of the bones of the roof are not difficult to decipher, for the eye detects 
quickly the parietals, frontals, and squamosals (Plate 37, fig. 1, p.,f, sq.); but nearly 
the whole extent of the rostrum has to be traversed before we reach the true nasals. 
These bones (n.) are small crescentic scutes that cover the small, distal olfactory 
capsules (ol.) But along the top of the rostrum, from the ethmoidal region, where the 
frontals diverge nearly to the nasal roofs, two long, narrow styles of bone are seen ; 
these I propose to call “ ethmo-nasals ” ( et.n .) ; they are manifestly separate centres 
that correspond to the elongated hinder part of the nasals of a Bird. 
The olfactory sac, in both Ganoids and Teleosteans, is devoid of a proper paraneural 
roof, and the bone covering it is merely one of the many “ slime-bones ” seen in the 
skulls of these Fishes ; still, that scute which directly covers the olfactory organ has 
the first claim to be called the nasal. The frontals ( f ) in their foremost third 
are divarcated and styloid, embracing the hind end of the etlimo-nasals {et.n.) ; they 
are wide where they meet over the ant orbital region, become pinched up to their 
hinder fourth, and then widen most where they are overlapped by the parietals. 
These latter bones (fig. 2, p.) are large and oblong, covering the skull well from the 
middle of the eye-balls to the back of the ear capsule ; they are flanked and overlapped 
by the temporal series. 
The principal temporal bone is the squamosal {sq.); it is a long and irregular triangle, 
with its sharp end foremost; its broad end is overlapped by the second large temporal 
{s.t.), which covers the hinder part of the parietal as a rounded scale. Under it there 
is a lesser pair, and under these upper, larger bones, there is a considerable patch of small 
scutes margined by the circumorbital series in front, and the angulated interopercular 
