Fishes of the Western North Atlantic 75 



at a time, feeding on garbage thrown overboard. However, the great majority are con- 

 fined to comparatively shallow water. While a few, which are mentioned below in the 

 appropriate connections, find their homes on the continental slopes at depths of some hun- 

 dreds of fathoms, the greatest depth for which there is definite record of the capture 

 of a shark of any species is about 1,500 fathoms. Nor is it likely that any shark is a 

 regular inhabitant of the floor of the oceanic abyss. The group is cosmopolitan, but the 

 great majority inhabit the tropical-subtropical belt. Characteristically temperate species 

 are much fewer in numbers, and only one genus (Somniosus) is a regular inhabitant of 

 truly polar waters. 



Classification. The question of how to subdivide the modern sharks so as to illustrate 

 the supposed phylogenetic relationship of diflFerent groups, which has been argued since 

 the days of the early comparative anatomists, is one that we pass over briefly. 



The paleontologic history of the groups of sharks that still exist throws little light on 

 the matter. Groups as diverse as the heterodonts, orectolobids, galeoids and squatinoids 

 were all in existence as far back as the Upper Jurassic, and the hexanchids were present 

 in the middle Jurassic and the squaloids in the Cretaceous 5 while "by the beginning of the 

 Tertiary period all of the living families of Elasmobranchs appear to have come into 

 existence."" 



Students of living sharks have agreed generally that the most primitive are those 

 (Hexanchidae and Chlamydoselachus) in which the vertebrae are calcified but weakly, 

 if at all, and in which the notochord is but little constricted segmentally. The hexanchids 

 likewise appear to agree with the Mesozoic genus Hybodus both in these features, and 

 further, in the mode of suspension of the upper jaws (p. 78 ) . However, if these supposedly 

 primitive groups were actually derived from the hybodoids, as has been suggested, they 

 have diverged widely from the ancestral stem by a multiplication of gill arches {Hybodus 

 had five only), by the loss of the second dorsal fin and of fin spines, and by modification in 

 their dentition. On the other hand the heterodonts, which resemble the ancient hybodoids 

 so closely in dentition, in number of gills and in the presence of two dorsal fins and fin 

 spines that they have often been united with them in a single suborder, differ from the 

 hybodoids in having the vertebrae well calcified, the notochord strongly constricted seg- 

 mentally and the upper jaw (palatoquadrate cartilage) attached to the cranium in one re- 

 gion only, without the postorbital connection which has often been regarded as primitive." 



Among the remaining, and far more numerous, living members of the order, much 

 weight in classification has been given to the degree and arrangement of the internal calci- 

 fications of the ve'-tebral centra. These centra may consist of only a primary ring surround- 

 ing the notochord ("cyclospondylic"), or of a primary ring with secondary calcifications as 

 well, either in concentric rings around the primary one or in bars (simple or branched) 



12. Romer, Vert. Paleont., 1933: 54- 



23. See De Beer (Devel. Vert. Skull, 1937: 421-4.25) for definitions of the rather complex terminology that has 

 been employed to define the different methods of attachment of upper jaw to skull. 



