HARDWICKE'S SCIENCE-GOSSIP. 



173 



dependent — to constitute each a distinct order — and 

 the structure of the skull is particularly considered to 

 justify this new classification. 



The entire skeleton of the Plagiostomes is cartila- 

 ginous, and, as might be inferred from what has 

 already been stated, the only hard parts capable of 

 fossilisation are teeth, spines, shagreen, and some of 

 the more fibrous and calcified cartilage. The skull is 

 peculiar and characteristic, and specially noticeable 

 on account of the arrangement of the jaws, which are 

 provided with numerous teeth. Instead of the upper 

 dentition being borne by maxillaries and premaxil- 

 laries, the palatines constitute the upper "jaw," and 

 these cartilages do not coalesce with the rest of the 

 skull, but, together with the mandible, form a de- 

 cided "arch." This is suspended by an element 



[Fig. loi. — Dorsal aspect of Ray (Raja), much reduced in size. 

 a, spiracle ;/, eye. 



(j) that is probably homologous with the hyoman- 

 dibular + symplectic of the Teleostean fishes, and is 

 diagrammatically represented in fig. 99. Two " labial 

 cartilages " on each side (not shown in the figure) are 

 probably homologous with the maxillaries and pre- 

 maxillaries of other fishes. The teeth vary in form 

 considerably, not only in the different genera and 

 species, but also in different parts of the individual 

 mouth ; sometimes they are more or less flat and 

 smooth, adapted for crushing food ; in some genera, 

 they are only slightly flattened, and ornamented with 

 grooves and ridges of varying fineness and complexity ; 

 while, in other genera, they are more or less conical 

 and laterally compressed, with a sharp cutting edge, 

 either serrated or entire, and particularly fitted for 

 grasping or piercing or for lacerating flesh. But, 

 however varied the teeth may be in general shape 



and arrangement, they all agree in possessing well- 

 defined roots, not lodged in sockets, but enveloped in 

 a somewhat elastic membrane, and thus attached to 

 the jaw ; those sharks with cutting teeth usually have 

 only one row in use at a time, and, notwithstanding 

 their loose implantation, are not able to erect or 

 depress them at will. Among living forms, at least, 

 the new teeth are developed behind the old, and 

 advance forwards to take the place of those that are 

 continually falling out, and there are only extremely 

 few cases among fossil forms where this plan of suc- 

 cession appears to be altered. A little behind the 

 head, on each side, are observed five narrow slits 

 (fig. 100, c), which constitute the external gill-open- 

 ings, and are not covered by 'any fold of skin ; the 

 "spiracle" (fig. loo, a), occasionally present, is also 

 interesting, — being the remnant of a sixth (really the 

 first) branchial cleft in the embryo, — and is the ex- 

 ternal opening of a small canal connected with the 

 pharynx. The body of the Plagiostomes, though 

 sometimes naked, is generally more or less protected 

 with bony granules or "shagreen," and, in a few 

 rays, these dermal ossifications are modified into 

 large spinous tubercles. The dorsal fins are frequently 

 armed in front with strong spines (fig. loo, 3,, h^, and 

 such spines, barbed, occur upon the tails of some 

 genera of rays without any accompanying fin. There 

 are still other most important characters, displayed in 

 the internal soft parts of these fishes — brain, heart, 

 intestine, &c. — but these are necessarily unavailable 

 to the palaeontologist, and space prevents us from 

 considering them here. 



The two sections of the Plagiostomi are easily re- 

 cognised ; the sharks (Selachoidei) are usually 

 elongated in shape (fig. loo), with a cylindrical body, 

 and the gill-openings lateral, while the rays (Batoidei) 

 are almost all much depressed and flattened (fig. loi), 

 with very large pectoral fins, and the gill-openings 

 on the ventral surface. There are, however, living 

 forms that appear to be intermediate between the 

 two sections, and several fossil species are known 

 that seem to be even more remarkable in this respect. 



The sharks and rays of the present day are mostly 

 marine, although some of the former often ascend 

 large rivers to a considerable distance from the mouth, 

 while a few species of each are found exclusively in 

 fresh water. All (except perhaps one) are carnivorous, 

 and those with flat crushing teeth feed upon molluscs, 

 Crustacea, and other creatures not requiring any very 

 rapid movement for their capture, while those with 

 sharp cutting teeth are much more agile and live 

 chiefly upon fishes. The species of sharks have a 

 very wide geographical range j some are true pelagic 

 forms, but the majority exist at no great distance 

 from the shore, and scarcely any have been met 

 with in the oceanic depths. The species of rays have 

 not so wide a geographical range as the sharks, and 

 only very few venture to leave the comparatively 

 shallow waters of the coast-line. 



