short in relation to length of body but conspicuous by its elongate, compressed, tapering snout with numerous fang–like teeth and large gape. Reduced, fleshy, flap-like pectorals present. Dorsal fin represented by a low fold without rays; anal and caudal fins not represented. Vent placed subterminally. Myomeres numerous, V-shaped, distinct along the anterior two-thirds of the body, less conspicuous posteriorly and not apparent from shortly before the vent to the tip of the tail.

Head 49 in total length, acutely tapering and compressed before the eye, snout 1.6 in head; dorsal profile of snout almost straight, not concave, with rostral cartilage forming a slight swelling in the dorsal profile just in advance of the eye. Jaws extended, compressed, acute anteriorly, bearing 22 to 24 teeth on each maxilla and 20 to 21 on each dentary. Teeth obtusely triangular in section with the base of the triangle medial; anterior 14–16 teeth on each element tall, acutely tapering, fang-like and tending to be directed forwards and outwards, especially the first eight; remaining teeth on each element similar in section but smaller and progressively reduced towards angle of gape. Three to four of the posterior teeth on the dentary are doubled so as to present a medially-directed and a laterally-directed tooth. Tip of lower jaw bearing a pair of anteriorly-directed fangs larger than any of the succeeding teeth (these are not included in the count of teeth previously given). Articulation of lower jaw just in advance of the level of posterior margin of eye; eye not telescopic, horizontal diameter 5.2 in snout, clearly less than the vertical. A large nasal organ distinct just in advance of the eye. Branchiostegal rays not developed. Branchial apertures about two-thirds of the way along the postorbital length, lateral, oblique, confluent below, free from the isthmus, extending to about the midlateral level on the head. Vent distinct, placed sub-terminally only 10 mm from the tip of the caudal region.

Dorsal fin present as a low fold, without rays, originating rather indeterminately some distance behind the level of the pectoral fin, that is, at about the level of the 45th myomere; anal and caudal fins not represented; pectoral fin a minute, reduced, fleshy flap with numerous delicate actinotrichi.

Established myomeres 466 counted, but a space equivalent to 20 myomeres from shortly before the level of the vent to the tip of the tail lacks myomeres, so that the fully-developed larva may have at least 486; myomeres V-shaped with only the extreme dorsal and ventral ends of the myosepta turning slightly forwards. Angle between dorsal and ventral halves of each segment obtuse anteriorly, diminishing posteriorly so that at the level of the 200th segment the angle is about 80°. Myomeres about 1.75 mm wide at the 75th segment, wider in the middle of the body where they are about 2.25 mm wide at the 200th myomere, progressively narrower posteriorly.

Body translucent in formalin, with pigment confined to a regular, ventrolateral series of grouped chromatophores before the level of the vent and an irregular series postanally. The anterior 15 groups of chromatophores before the level of the 45th myomere are separated by intervals of three segments, are bandlike in appearance, each band placed transversely across the ventral aspect, continuing on to the ventrolateral aspect and not extending above the gut. The succeeding 40 groups of chromatophores to the level of about the 210th myomere are separated by intervals of about four segments; along this section there is a tendency for the anterior groups to continue the banded appearance of those of the preceding section but posteriorly the pairs of grouped chromatophores on each ventrolateral aspect fail to coalesce ventrally and intervening smaller groups are present. The latter gradually increase in size relative to the former until the groups are very nearly equal in size near the vent. From the 210th myomere to the 300th, the groups of chromatophores are separated by intervals of three myomeres; posterior to this by about two segments. Postanally there are 19 separated chromatophores, irregularly spaced. On the head one large chromatophore is placed halfway along the lateral surface of the right mandible, a smaller one on the left. A few small chromatophores are scattered over the branchial region on the right side. No lateral line pores are present and lateral line organs are not apparent.

The posterior position of the vent, the lack of fin-rays, the numerous larval teeth, the single naris and the abundance of larval pigment indicate that the specimen of Leptocephalus giganteus is at an early stage in larval development as is shown also by the general lack of ossification in the skeleton. The entire neurocranium and most of the splanchnocranium are cartilaginous with vertebral column and supporting elements of the median fins not apparent. The elongate rostral cartilage seems massive, makes up the major portion of the snout, and is excavated posterodorsally to receive the large, undivided nasal organ on either side. Roofing the cranial vault there is a solid plate of cartilage continuous anteriorly with the rostral cartilage and laterally with the rather indistinct supraorbital ridges. On the posterolateral aspect of the neurocranium a large and conspicuous otic capsule is developed, but the external horizontal semicircular canal is distinct. Except for the rectangular quadrate which articulates directly with meckel's cartilage anteriorly, the cartilaginous upper jaw cannot be recognised and may be reduced or absent as in adult Nemichthyidae. Small, paired, ossified premaxillae are firmly attached to the anterior end of the rostral cartilage. The maxillae are

ossified as thin sheets on either side of the rostrum and bear the conspicuous larval teeth of the upper jaw. A large meckel's cartilage makes up the major portion of the lower jaw, but is overlaid dorsolaterally by the narrow, thinly-ossified dentary bearing larval teeth. There is no obvious distinct articular element. The hyoid arch is represented by the hyomandibula, the epihyal and the ceratohyal with basihyal not observed; the hyomandibula is broadly triangular, directed obliquely forwards and perforated by the foramen for R. hyomandibularis N. VII. Opercular elements are not obvious. The branchial skeleton is cartilaginous, but nevertheless almost fully represented with gill filaments present on the epi- and cerato-branchials. The fifth branchial arch is reduced. A thin, elongate, curving cleithrum is present just in advance of the pectoral flap.

The muscular system is well developed at this stage. The large m. adductor mandibulae originates over the anterior third of the lateral surface of the hyomandibula and the upper half of the quadrate and is inserted on to the posterodorsal angle of the meckel's cartilage. Just above the most dorsal extremity of this muscle lies a small m. adductor arcus palatini and posterior to this the smaller m. dilator operculi rising from preotic and mesotic origins respectively. The eye muscles are distinct. The mm. levatores branchiales are conspicuous and consist of four elongate, narrow, external levators and one internal levator, this latter being attached to the first branchial arch. Ventral muscles present are the m. inter-mandibularis and the m. sternohyoideus (= claviculohyoideus). The latter has not yet established an observable skeletal origin.

With the exception of the tractus opticus, the nn. oculomotores and the n. opticus which are obscured and cannot be located, the cranial nerves are immediately apparent in lateral and dorsal views. T. olfactorius extends forwards above the eye as a thick trunk dividing terminally into two ramifying branches on the as yet undivided nasal organ. Nn. trigeminalis and facialis arise close together just behind the eye, N.V. dividing proximally into five branches. (1) R. cutaneous quinti, a short nerve passing dorsally, (2) R. orbitonasalis, passing forwards above the eye and nasal organ, (3) R. maxillaris, a long and much-divided nerve to the snout and maxilla, (4) R. mandibularis, to the posterior margin of meckel's cartilage, and (5) R. palatonasalis (distally obscured), to the parasphenoid region; N. VII is proximally obscured, but the perforation of the hyomandibula by the foramen for this nerve is clear, the nerve sending short branches to the hyoird arch and a long branch (R. mandibularis N. VII) to the lower jaw. N. glossopharyngeus is first apparent posterior to the levator muscles of the branchial arches and carries a small ganglion half-way along its path to the first branchial arch. The branches of N. vagus are conspicuous, and all except the innervation of the reduced fifth arch are ganglionated shortly after the exit of the vagus from the cranium. R. visceralis N. X turns ventrally at the level of the fifth arch to pass along the dorsal aspect of the oesophagus; R. lateralis N. X is thicker and can be traced to the level of about the 35th myomere. The spinal cord is rather indistinct but it can also be traced to about the same level.

Text-fig. 2. —Leptocephalus giganteus n. sp. Type, 893 mm T. L. Lateral view of head region to show major anatomical structures. aap, m. adductor arcus palatini; c. chromatophore; ch, ceratohyal; d, edge of dentary; do, m. dilator operculi; eh, external horizontal semi-circular canal; h, heart; im, m. intermandibularis; lb, mm. levatores branchiales; no, nasal organ; q, quadrate; r, edge of rostral cartilage; re, m. rectus externus; sh, m. sternohyoideus; N. IX, N. glossopharyngeus.