ESPECIALLY MYCTOPHOIDS 199 
more likely to have been pelagic and not benthonic. The forward migration of the 
pelvics in Phylactocephalus would seem to have served a different function to that of 
Synodus, and may possibly be analogous in function to the advanced pelvics of the 
acanthopterygians. In these the forwardly placed pelvics cancel out the lift force 
produced by the pectorals when they are used as brakes (Harris 1938 : 37 ; 
Patterson, 1964 : 452). 
The structure of the jaws is identical in Halec (Text-fig. 61), Phylactocephalus and 
Hemisaurida (Text-fig. 67) providing the most obvious link between the three 
genera. The premaxillae are simple flattened plates with no articular or ascending 
processes. The oral border is toothed and forms half of the gape. The maxilla is 
thin, narrow and elongated, entering the gape behind the premaxilla. The head of 
the maxilla is little differentiated but does appear to be associated with a small 
palatine process. The teeth on the maxilla are of characteristic form, larger than 
those on the premaxilla and inclined forwards. A single, small ornamented supra- 
maxilla is associated with the posterior end of the maxilla. 
The dermal upper jaw is unspecialized and primitive but with certain advanced 
tendencies such as extension beneath the maxilla of the premaxilla, narrowing of the 
maxilla, and loss of one supramaxilla. 
Apart from the jaws the genus Hemisaurida (Text-figs. 67 and 68) shows several 
features not seen in either Halec or Phylactocephalus. The head and body remain 
shallow (as in Halec) but the snout has become elongated. Coupled with this 
elongation the jaw articulation has moved forwards to a point below the hind end 
of the orbit. Thus the suspensorium is inclined forwards. Ventrally the pre- 
operculum in Hemisaurida is produced into a prominent backwardly projecting 
spine (a similar spine has already been noticed in the ichthyotringoid A pateopholis 
Text-fig. 15, but in neither can any functional correlation be attached to it). The 
pectoral fin, like that of Halec and Phylactocephalus, is low on the flank, but is 
characterized by extension of certain of the rays. These extended rays are long 
and filiform, and the fin is not aliform as in the Exocoetidae or the Cretaceous 
Chirothricidae (Woodward, Igor : 281, fig. 10). In both of these latter groups the 
pectorals are spread into wing-like structures and are positioned quite high on the 
flanks. The body of Hemisaurida is devoid of squamation, whereas both Halec 
and Phylactocephalus possess a complete covering of small scales. 
Although the caudal skeleton is not well represented in the three genera, sufficient 
can be made out to indicate that itis comparable (Halec, Text-fig. 64 and Hemisaurida, 
Text-fig. 69). The first preural vertebra is fused with the first ural vertebra and this 
compound centrum supports a stegural dorsally. The second ural centrum is much 
reduced and appears to support the four hypurals to the upper caudal lobe. The 
posterior haemal spines have a tendency to become laterally compressed. 
A feature of both Halec and Phylactocephalus, not seen in Hemisaurida, is the 
expanded nature of the bases of the anterior fin rays in all fins except the caudal. 
These expanded basal regions are ornamented. This feature is not normally 
associated with soft rays, but is occasionally found on the fin spines of Acantho- 
pterygii. 
