. Bulletin of the British Museum (Natural History) Zoology. THE HALOSAUR LEPTOCEPHALUS 455 was slightly smaller (430 mm.) than previously described species and is perhaps a species generally characterized by small size. One is led, further, to wonder whether interorbital width in Aldrovandia does not increase with age. The levator arcus-palatini/hyoidei muscles which in Aldrovandia (as in Polypterus) slant forward and upwards to insert on the frontals, are placed more vertically in Halosaurus. The greater interorbital width seems related to the larger surface required for the muscle insertions
. Bulletin of the British Museum (Natural History) Zoology. THE HALOSAUR LEPTOCEPHALUS 455 was slightly smaller (430 mm.) than previously described species and is perhaps a species generally characterized by small size. One is led, further, to wonder whether interorbital width in Aldrovandia does not increase with age. The levator arcus-palatini/hyoidei muscles which in Aldrovandia (as in Polypterus) slant forward and upwards to insert on the frontals, are placed more vertically in Halosaurus. The greater interorbital width seems related to the larger surface required for the muscle insertions in Aldrovandia. Perhaps, too, the wider spacing of the supraorbital lateral-line canals, concomitant with wider frontals, explains the differences in squamation. Scales are developed in Halosaurus in which the canals are close together, but not in Aldrovandia in which they are wider apart. The larva under discussion has developing scales only in the giant suborbital and mandibular canals, so that one cannot rely on this character here. The levator arcus palatini muscles (Text-fig. 5) are placed almost vertically and originate on the posterior border of the orbit and the lateral wall of the cranium. The supra- orbital canals, on the other hand, are not very close together, running almost along the upper rims of the orbits. One might perhaps expect a broadening of the head from the compressed state pre-supposed in a leptocephalus head. Also, the origins of the levator arcus palatini muscles are narrow crescents on the frontals of a small syntype of A. phalacra, suggesting that the muscle may increase in bulk during P°P- sop" km. Fig. 5. Head of Aldrovandia leptocephalus showing musculature, = nasal capsule, = levator arcus palatini, dil. op. = dilator operculi, = levator operculi, op. = operculum, = suboperculum, = preopercular flange, pop. = pre- operculum, i. op. = interoperculum, sor. = suborbital lateral line canal.
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