56 



MARINE AND FISHERIES 



1-2 EDWARD VII., A. 1902 



point of view it is hardly necessary to point out that very diverse views are held 

 respecting the significance of these cartilages and the process by which they assumed 

 their present form and arrangement. Indeed, as Professor Wiedersheira has said,f " No 

 other morphological problem has given rise, during the last twenty years, to such exten- 

 sive researches, and to such varied solutions as the question of the origin of the paired 

 limbs. Two very opposite views exist. According t,o one of these (Gegenbaur's view) 

 the proximal parts of the extremities, that is, the pectoral and pelvic arches, are regarded 

 as being derived from branchial arches, and the distal or free portions as metamorphosed 



fin rays According to the other view (that of Dohrn), the origin of the paired 



limbs has nothing to do with the visceral skeleton : but, like the latter, they are to be 

 looked upon as the localized remains in definite regions of the body (thoracic and pelvic 

 regions) of a series of cartilaginous bars extending originally along the whole trunk, and 

 having a metameric arrangement. In other words, just as each body-segment of an 

 Annulate may be looked upon as being provided with a pair of limbs, so also was each 

 primitive segment of the Vertebrate body ; recent researches seem to support this.' 

 Professor Huxley adopted Gegenbaur's theory, though with grave modifications, and 

 the theory of Dr. Anton Dohrn has been considerably transformed by the researches 

 and suggestions of Mivart, F. M. Balfour, and J. K. Thatcher. Whatever be the mode 

 of origin of the limbs of fishes they present in Plagiostomes, the Holocephali, and other 

 primitive forms, certain structural features in common, and in most of them the tripartite 

 nature of the basal cartilages is clearly seen. One or more may abort or maybe shifted 

 from direct articulation with the pectoral bar ; but one (according to Gegenbaur the 

 metapterygium ; according to Huxley the mesopterygium) is constant, and through it 

 the theoretical axial line of the limb must be drawn. It is clear that an element of 

 uncertainty must often attach to the determination of these basal cartilages, but the 

 same is true of even so familiar an extremity as the frog's manus, for the middle 

 element of the proximal row of ossa carjmlia is named by Ecker the os lunatum, whereas 

 Duges did not hesitate to pronounce it the os naviculare. 



But, as already stated, ther<) is a uniformity in the basal elements present in these 

 primitive forms of the locomotor limb, and the comparison of a large number of diverse 

 types, illustrated in the existing species of Plagiostomes, Ganoids, ko., affords a guide 

 to their accurate interpretation. 



SKELETON OF THE FIN. 



The fin of Lamna is in many respects peculiarly interesting. On comparing the 

 number, form and disposition of the skeletal elements, with those seen in the fins of 

 other primitive types of fishes, we observe a number of noteworthy morphological 

 features. In the first place the basal pieces (Plate V., fig. 1, pro. mesop. metap.) are not 

 lengthened and expanded as in Acanthias (Plate VII., fig. 4) or Scylliiim (Plate VII., 

 fig. 3) but form a row of compact shortened elements, of which the metapterygium 

 (metap.) alone is somewhat elongated, though in the lateral direction, not in the longi- 

 tudinal as seen in the fins of the species just referred to. Now the whole fin expansion, 

 is enormously lengthened longitudinally, and this shortening in the length of the basal 

 pieces results in the exaggerated enlargement of the remaining part of the cartilaginous 

 skeleton. The rows of jointed rays, whose extent is so much reduced in Acanthias, in 

 Heptanchus (Plate VII., fig. 5) though so primitive a form, and in Chimaera and Folyodon 

 (Plate VII., figs. 6 and 8) are in Lamna so long and cover transversely so large a space 

 that they are almost coterminous with the entire outer limits of this extensive lobate 

 paddle. Upon the outer portions of the cartilaginous expanse the thick provision of 

 slender horny rays forms a dense thatch, and extends only for a short distance beyond 

 the distal margin of the radial elements (Plates V. and VI., figs. 1 and 2,h.). Fully seven- 

 eighths of the fin-expansion are occupied by these jointed rays, the basal plates covering 

 less than one-eighth of the surface of the fin, though in most Selachian fins, they cover 

 proportionally three or four times that area. There has been reduction in the length of 



t Elements of the Comp. Anat. of Vertebrates, trans, by W. N. Parker, London, 1866, p. 86. 



