374 
MESSRS. P. M. BALFOUR AND W. N. PARKER ON THE 
The pelvic fins are somewhat more prominent. 
The yolk-sac, as pointed out by Agassiz, has now disappeared as an external 
appendage. 
After the stage last described the young Fish rapidly approaches the adult form. To 
show the changes effected we have figured the head of a larva of about a month old 
and 23 millims. in length (Plate 21, fig. 15). The suctorial disc, though much reduced, 
is still prominent at the end of the snout. Eventually, as shown by Agassiz, it forms 
the fleshy globular termination of the upper jaw. 
The most notable feature in which the larva now differs in its external form from 
the adult is in the presence of an externally heterocercal tail, caused by the persistence 
of the primitive caudal fin as an elongated filament projecting beyond the permanent 
caudal (Plate 28, fig. G8). 
Delicate dermal fin-rays are now conspicuous in the peripheral parts of all the per¬ 
manent fins. These rays closely resemble the horny fin-rays in the fins of embryo 
Elasmobranchs in their development and structure. They appear gradually to enlarge 
to form the permanent rays, and we have followed out some of the stages of their 
growth, which is in many respects interesting. Our observations are not, however, 
complete enough to publish, and we can only say here that their early development 
and structure proves their homology with the horny fibres or rays in fins of Elasmo- 
branchii. The skin is still, however, entirely naked, and without a trace of its future 
armour of enamelled scales. 
The tail of a much older larva, 11 centims. in length, in which the scales have begun 
to be formed, is shown in Plate 21, fig. 16. 
We complete this section of our memoir by quoting the following passages from 
Agassiz as to the habits of the young fish at the stages last described :— 
“ In the stages intervening between plate iii, fig. 19, and plate iii, fig. 30, the young 
Lepidosteus frequently swim about, and become readily separated from their point of 
attachment. In the stage of plate iii, fig. 30, they remain often perfectly quiet close to 
the surface of the water; but, when disturbed, move very rapidly about through the 
water. . . . The young already have also the peculiar habit of the adult of coming 
to the surface to swallow air. When they go through the process under water of 
discharging air again they open their jaws wide, and spread their gill-covers, and 
swallow as if they were choking, making violent efforts, until a minute bubble of air 
has become liberated, when they remain quiet again. The resemblance to a Sturgeon 
in the general appearance of this stage of the young Lepidosteus is quite marked.'" 
Brain. 
I. Anatomy. 
The brain of Lepidosteus has been figured by Busch (whose figure has been copied 
by Miklucho-Maclay, and apparently by Huxley), by Owen, and by Wilder 
