STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
373 
In the region of the head there is a considerable elongation of the pre-oral part, 
forming a short snout, at the end of which is placed the suctorial disc. At the sides 
of the snout are placed the nasal pits, which have become somewhat elongated 
anteriorly. 
The mouth has lost its open rhomboidal shape, and has become greatly narrowed in 
an antero-posterior direction, so that its opening is reduced to a slit. The mandibles 
and maxillary processes are nearly parallel, though both of them are very much 
shorter than in the adult. The operculum is now a very large flap, and has extended 
so far backwards as to cover the insertion of the pectoral fin. The two opercular 
folds nearly meet ventrally. 
The yolk-sac is still mote reduced in size, one important consequence of which is 
that the pectoral fins (pc.f) appear to spring oiit more or less horizontally from the 
sides of the body, and at the same time their primitive line of attachment to the body 
becomes transformed from a longitudinal to a more or less transverse one. 
The first traces of the pelvic fins are now visible as slight longitudinal projections 
near the hinder end of the yolk-sac (; pl.J 
The pigmentation marking the regions of the permanent fins has become more pro¬ 
nounced, and it is to be specially noted that the ventral part of the caudal fin (the 
permanent caudal) is considerably more prominent than the dorsal fin opposite to it. 
The next changes, as Agassiz points out, “ afe mainly in the lengthening of the 
snout; the increase in length both of the lower and upper jaw; the concentration of 
the sucker of the sucking disc; and the adoption of the general colouring of some¬ 
what older Fish. The lobe of the pectoral has become specially prominent, and the 
outline of the fins is now indicated by a fine milky granulation. Seen from above, the 
gill-cover is seen to leave a large circular opening leading to the gill-arches, into which 
a current of water is constantly passing, by the lateral expansion and contraction of 
the gill-cover; the outer extremity of the gill-cover covers the base of the pectorals. 
In a somewhat older stage the snout has become more elongated, the sucker more 
concentrated, and the disproportionate size of the terminal sucking-disc is reduced; 
the head, when seen from above, becoming slightly elongated and pointed,” 
In a larva of about 18 days old and 21 millims. in length, of which we have not given 
a figure, the snout has grown greatly in length, carrying with it the nasal organs, the 
openings of which now appear to be divided into two parts. The suctorial disc is 
still a prominent structure at the end of the snout. The lower jaw has elongated 
correspondingly with the upper, so that the gape is very considerable, though still 
very much less than in the adult. 
The opercular flaps overlap ventrally, the left being superficial. They still cover 
the bases of the pectoral fins. The latter are described by Agassiz as being “ kept 
in constant rapid motion, so that the fleshy edge is invisible, and the vibration seems 
almost involuntary, producing a constant current round the opening leading into the 
cavity of the gills.” 
