The general form of Lepidosteus shortly after hatching is shewn in fig. 67. On the ventral part of the front of the head is placed the large suctorial disc. At the side of the head are seen the olfactory pit, the eye and the auditory vesicle; while the projecting vesicle of the mid-brain is very prominent above. Behind the mouth follow the visceral arches. The mandibular arch (_md_) is placed on the hinder border of the mouth, and is separated by a deep groove from the hyoid arch (_hy_). This groove is connected with the hyomandibular cleft, but I have not determined whether it is now perforated. The posterior border of the hyoid arch is prolonged into an opercular fold. Behind the hyoid arch are seen the true branchial arches. [FIG. 67. LARVA OF LEPIDOSTEUS SHORTLY AFTER HATCHING. (After Parker.) _ol._ olfactory pit; _op._ optic vesicle; _au.v._ auditory vesicle; _mb._ mid-brain; _sd._ suctorial disc; _md._ mandibular arch; _hy._ hyoid arch with operculum; _br._ branchial arches; _an._ anus.] There is still a continuous dorso-ventral fin, in which there are as yet no fin-rays, and the anterior paired fins are present. The yolk-sack is very large, but its communication with the alimentary canal is confined to a narrow vitelline duct, which opens into the commencement of the intestine _immediately behind_ the duct of the liver, which is now a compact gland. The yolk in Lepidosteus thus behaves very differently from that in the Sturgeon. In the first place it forms a special external yolk-sack, instead of an internal dilatation of part of the alimentary tract; and in the second place it is placed behind instead of in front of the liver. I failed to find any trace of a pancreas. There is however, _opening on the dorsal side of the throat_, a well-developed appendage continued backwards beyond the level of the commencement of the intestine. This appendage is no doubt the air-bladder. In the course of the further growth of the young Lepidosteus, the yolk-sack is rapidly absorbed, and has all but disappeared after three weeks. A rich development of pigment early takes place; and the pigment is specially deposited on the parts of the embryonic fin which will develop into the permanent fins. The notochord in the tail bends slightly upwards, and by the special development of a caudal lobe an externally heterocercal tail like that of Acipenser is established. The ventral paired fins are first visible after about the end of the third week, and by this time the operculum has grown considerably, and the gills have become well developed. The most remarkable changes in the later periods are those of the mouth. [FIG. 68. HEAD OF AN ADVANCED LARVA OF LEPIDOSTEUS. (After Parker.) _ol._ openings of the olfactory pit; _sd._ remains of the larval suctorial disc.]