20 4 ELOPIFORM FISHES 



the lower margins of the rami meet in the ventral mid-line. The skin between the 

 jaw rami is folded in the Recent albuloids and in Albula the gular plate, which is 

 necessarily reduced in size, lies vertically. The Pterothrissidae have lost the gular plate. 



The sensory canal system of the snout in the Albulidae and Pterothrissidae is 

 peculiarly modified and provides a possible link with the Notacanthiformes. Primi- 

 tively, as in 0. lewesiensis, the infraorbital canal ran from the first infraorbital to the 

 dermethmoid where it joined its partner of the opposite side through the ethmoid 

 commissure. Although they have not been found, it is assumed that there were 

 canal-bearing rostral ossicles between the dermethmoid and the first infraorbital in 

 0. lewesiensis. In the Albulidae and Pterothrissidae the snout has turned downwards 

 and the ethmoid commissure, no longer in its primitive position, is not a continuous 

 transverse canal. Rather the infraorbital canal now descends through rostral 

 ossicles on to the premaxilla. The extension of the infraorbital canal on to the pre- 

 maxilla is a development unique to albuloids and notacanthiforms. 



The premaxilla of 0. latifrons is unknown but it is suspected that canals were 

 developed on the premaxilla in this form among the Osmeroididae. 



In addition to the modified infraorbital canal, in the Pterothrissidae the supra- 

 orbital sensory canals of either side unite across the dermethmoid. In Albula the 

 supraorbital sensory canals are separated anteriorly by a median cavity which is 

 isolated from the canals on either side by membrane (Gosline 1961). The similarity 

 of this condition to that in anguilliform fishes has been noted by Gosline (1961). 



In the majority of ' lower ' teleostean fishes the sensory canals open to the surface 

 by large pores. In the Albulidae and Pterothrissidae, however, the canals are 

 covered by taut skin perforated by many tiny pores. The canals themselves are 

 probably filled with water and the taut skin acts like a tympanum, resonating in 

 sympathy with impinging vibrations. With the exception of the supraorbital, otic 

 and temporal canals, the sensory canals are directed ventro-laterally and ventrally 

 and accord with the benthic habit of the albuloids. 



There are many other fishes with a cavernous canal system. The majority are 

 inhabitants of deep water (e.g. Macrouridae), but there are also shallow-water forms 

 (e.g. Notopteridae). The presence of a cavernous sensory canal system is therefore 

 not necessarily related to habitat, and while its significance (other than perhaps 

 increasing sensitivity) is not understood, its advanced nature is undoubted. 



With the slightly elongated snout and the short, anteriorly situated lower jaw, the 

 hyopalatine apparatus of the Albulidae and Pterothrissidae has become modified. 

 The hyopalatine series is long and shallow with the articular face of the quadrate 

 directed anteriorly. The shifting of the quadrate/mandibular articulation has been 

 achieved by forward displacement of the quadrate which is linked to the hyo- 

 mandibular by a somewhat lengthened and flattened symplectic. The metaptery- 

 goid has come to support the symplectic. A condition very much like that seen in 

 Recent albuloids is found in 0. latifrons (Text-fig. 59). 



A further development in the hyopalatine series of most albuloids is the large 

 foramen between the hyomandibular and the metapterygoid, through which pass 

 deeper fibres of the levator arcus palatini muscle. In 0. lewesiensis and 0. latifrons, 

 as in the elopoids and some other teleosts, the metapterygoid bears a ridge on the 



