STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
371 
disc with numerous papillae, of which we shall say more hereafter. This disc is some¬ 
what bifid, and is marked in the centre by a deep depression. 
Dorsal to it, on the top of the head, are two widely separated nasal pits. On the 
surface of the yolk, in front of the head, is to be seen the heart, just as in Sturgeon 
embryos. Immediately below the suctorial disc is a slit-like space, forming the mouth. 
It is bounded below by the two mandibular arches, which meet ventrally in the median 
line. A shallow but well marked depression on each side of the head indicates the 
posterior boundary of the mandibular arch. Behind this is placed the very conspicuous 
hyoid arch with its rudimentary opercular flap ; and in the depression, partly covered 
over by the latter, may be seen a ridge, the external indication of the first branchial 
arch. 
Eleventh day after impregnation: External characters. —The embryo (Plate 21, 
fig. 10) is now about 10 millims. in length, and in several features exhibits an advance 
upon the embryo of the previous stage. 
The tail fin is now obviously not quite symmetrical, and the dorsal fin-fold is con¬ 
tinued for nearly the whole length of the trunk. The suctorial disc (Plate 21, fig. 11, 
s.d.) is much more prominent, and the papillae (about 30 in number) covering it are 
more conspicuous from the surface. It is not obviously composed of two symmetrical 
halves. The opercular flap is larger, and the branchial arches behind it (two of which 
may be made out without dissection) are more prominent. 
The anterior pair of limbs is now visible in the form of two longitudinal folds pro¬ 
jecting in a vertical direction from the surface of the yolk-sac at the sides of the body. 
The stages subsequent to hatching have been investigated with reference to the 
external features and to the habits by Agassiz, and we shall enrich our own account 
by copious quotations from his memoir. 
He states that the first batch were hatched on the eighth* day after being laid. 
“ The young Fish possessed a gigantic yolk-bag, and the posterior part of the body 
presented nothing specially different from the general appearance of a Teleostean 
embryo, with the exception of the great size of the chorda. The anterior part, how¬ 
ever, was most remarkable; and at first, on seeing the head of this young Lepidosteus, 
with its huge mouth-cavity extending nearly to the gill-opening, and surmounted by 
a hoof-shaped depression edged with a row of protuberances acting as suckers, I could 
not help comparing this remarkable structure, so utterly unlike anything in Fishes or 
Ganoids, to the Cyclostomes, with which it has a striking analogy. This organ is also 
used by Lepidosteus as a sucker, and the moment the young Fish is hatched he 
attaches himself to the sides of the disc, and there remains hanging immovable; so 
firmly attached, indeed, that it requires considerable commotion in the water to make 
him loose his hold. Aerating the water by pouring it from a height did not always 
produce sufficient disturbance to loosen the young Fishes. The eye, in this stage, is 
* This statement of Agassjz does not correspond with the dates on the specimens sent to us_a fact no 
doubt due to the hatching not taking place at the same time for all the larvse. 
