ALEPISAUROID AND MYCTOPHOID FISHES 325 



On the other hand, abyssal benthic fishes might be expected to have poorly ossified skeletons, yet in 

 the Bathypteroidae, and Ipnopidae (to consider only the Iniomi), the skeleton is well formed and much 

 denser than in the alepisauroid fishes. (This is well shown in X-ray photographs.) A more solid 

 skeleton is no handicap to a fish which lives on the deep-sea floor, whereas reduced ossification would 

 appear to be an advantage to the bathypelagic Alepisauroidea. This fact compensates these active 

 predacious fishes for the absence of a swim-bladder, for it must result in the lowering of the sinking- 

 factor, and hence less energy is required to maintain the fish at a particular feeding-level in the ocean. 

 The correlation between the lack of a swim-bladder and the laying-down of less skeletal material in 

 bathypelagic fishes will be considered more fully in a later paper, but one comparison may be made 

 now. Gonostoma denudatum has a large, well-developed swim-bladder, while in G. bathyphilum there 

 is no trace of this organ. A specimen of denudatum (131 mm.) was placed alongside one of bathyphilum 

 (122 mm.) and an X-ray photograph obtained. This strikingly confirmed the impression formed by 

 handling these fishes, that denudatum had a better ossified skeleton than bathyphilum. 1 If this interpre- 

 tation comes near the truth, more striking corroboration would be hard to find than these structural 

 differences between the two species. 



Text-fig. 7. Part of the lateral line (near the origin of the adipose dorsal fin) of Anotopterus pharao, showing two of 

 the scale-like ossifications and the lateral line pores ( x 6-6). 



The setting of the pectoral fins. In defining the two suborders of the Iniomi, Regan ( 1 9 1 1 ) stated that 

 the pectoral fins were lateral in the Myctophoidea, but low in position in the Alepisauroidea. Certainly, 

 the pectorals of Alepisaurus, Anotopterus, Omosudis and Evermannella are placed very low on the 

 shoulders, while those of the Aulopidae, Chlorophthalmidae, Synodontidae, Harpadontidae, Ipnopi- 

 dae and Bathypteroidae are laterally placed. But a number of exceptions to these generalizations are to 

 be found in the Myctophidae, Scopelarchidae and Paralepididae. 



Measurements of the angle between the horizontal axis of the body and the axis of the pectoral fin 

 have indicated that such an index comes close to being a diagnostic feature of each suborder. In most 

 alepisauroids this angle is below 45 : in most myctophoids it is above 45 . Data, showing what may be 

 called the pectoral angle, are listed in Table 4. 



It is interesting to consider these data in the light of the ideas of Harris (1952) on the function and 

 the evolution of the fins in fishes. Harris contended that, with the evolution of a swim-bladder, there 

 was no longer any tendency for the body to sink or even tilt, because the centres of gravity of the body 

 and the swim-bladder are usually either close together, or else coincident. Pectoral fins set as hydro- 

 planes and an asymmetrical tail, both giving a lift-force (as in sharks) were thus no longer necessary. 

 So the pectorals were freed to become limbs in the Choanichthyes, and brakes (or oars) in the Teleostii, 

 while the caudal fin attained a symmetrical neutral shape, but with a different structural basis, in the 

 Dipnoi, Crossopterygii and Actinopterygii. With increasing specialization of the pectoral fins as 

 brakes, there appears to have been an upward movement of the fins along the sides of the fish and a 

 rotation of the axis of each fin so as to bring the axis near to a vertical position. 



Since the Alepisauroidea have no swim-bladder they might be expected to have a specific gravity 

 somewhat greater than their environment (see p. 324). In order to remain at one level, an alepisauroid 



1 Fishes of this size may contain well-developed ova, showing that they are nearing, or have attained, the adult phase. Thus 

 the differences in ossification are unlikely to be due to differences in the phase of life. 



