KL.SSMiiliKANCIl;- 



IOC? 



shii|)<'ii disk nf cartilauf (ti^-. 2'.l."), y*///) on <'ai-li side 

 of tli<' |)cl\is. uriiied witli sharj) tcctli at the uuicr 

 (lowci') iiiai'iiiii, and analogous in jiosition to the \n-.v- 

 |iiil)is of I lie ISalracliiaiis and Iteptilcs. 



The tin-i'a\'s of most of tlic l'">iasnioliranciis nvc 

 cartilaginous, distal sections (joints) of the rudialia : hut 

 ill the nuMiilirane of flu; outer parts of the fins — soiiie- 

 tiiiii's, as in tiie dorsal fins of the Kays, also between 

 till' tojis of tlie true rays - are set fibrillar or corneous 

 i'a\'s (figs. 2'.t4 — '2'M. fh), such as we Innc seen above 

 in the eiubrxdiiic fins of the Teleosts or in the so- 

 called adipose tin. In the Sharks and Chiniieras siniihir 

 i'a\s su)i|)orf th(-' greatei- part of the fins (outside the 

 radialia). Well-inarked calcifications indeed appear, 

 iiiidcr the form of hn-rays almost e\actl\' resembling 

 in their extcricjr the sj)inous rays we ha\(> seen among 

 tlie preceding fishes. Such i-avs are set in many Elas- 

 luohranchs before the unpaired fins (fig. :294, iktd); 

 and in some Kays they arm the tail with mobile wea- 

 pons of offence and defence. In pala'ontology the\" beai- 

 the name of icIifln/oiIoniUtes. Agassiz has shown", that 

 both as regards tiicir attachment, which is without true 

 articulation, and their texture, which exactly corre- 

 sponds to the dentinal structure of dei'inal spines (])la- 

 coid scales) and jaw-teeth, these spines are widely dif- 

 ferent from the spinous rays of the Teleosts. They are 

 mere dermal gi'owths; but inasmuch as they immediate- 

 ly overlap- and cover with their bases the antericjr 

 margin of the supporting cartilages of the fins — e. g. 

 the spines of the dorsal fin in the Sharks and Chiinie- 

 ras — they should also lie interpreted as most nearly 

 representing skeletal tegumentary bones, although they 

 are without true ossification. 



The skeleton of the Elasmobranchs is characterized 

 by this very want of endoskeletal and tegumentary hone. 

 Calcifications of great extent appear it is true, in old 

 specimens, under the form af a thin coat outside or 

 just within the surface of the skeletal cartilage, and in 

 several Sharks the intercellular mass of the vertebral 

 cartilages is radially or (at the centre of fiie vertelira) 

 concentrically calcified. But there is no true ossification, 

 riie structure of the S|iinal column, comjiosed of very 

 niiinerous vertebra^, displays the most sweeping altera- 

 tions and the most radical divergencies. In the VWi- 

 ina^ras tlie notochord (fig. 294, rh) remains almost un- 

 altered throughout the life of the fish; hut around it 



slender eaicilic rings ile\clop, sc\'ei-al, i'we or even more, 

 to eacli vertebra, whose area tiiay be defined as the space 

 between the origins of two pairs of roots Ijelonging to 

 the iiieiliillai'\ iier\es. In the Sharks and Mays, on tlie 

 other hand, ihe notochord is more or less consfrit^ted 

 dig. 2!lt'i, r), or <\en siiijiped olf entireh' at each ver- 

 tebra (tig. 297, (■), by the formation of the hollow 

 doulile cones, contiguous at the tops, of which the body 

 of the \ertehra is eoiiiposed. The neural arches of each 

 verteiira are usiialix iirokeii up into two or UKjre jjairs 

 of triangular disks, Ihe true iieura|ioph\-ses (figs. 294, 

 29(;, and 297, i/a) applying their base to the body of 

 the vertelira, and the other disks, the so-called upper 

 inter'.-alaria or iiitercriiralia (///), wedging the apex of 

 their triangle in a downward direction between the 

 neurapophj'ses. Tlie spinal canal is sometimes (fig. 294, 

 sp and fig. 297. parilv lietwecn /c) closed almve by 

 the develojiment cif un|iaire<l tt-rminal parts, answering 

 to the upper sjiiiious |irocesses of more developed ver- 

 tebra", and where the vertical fins appear, vertical disks of 

 cartilage sometimes afford high sui)|)orts to these lins and 

 their I'adialia — sometinies |»roduced by the coalescence 

 of the spinous processes, as in the ('hima'ras (tig. 294,?;), 

 sometimes corresponding to the inters])inal or interneural 

 bones of the Teleosts. In other cases this closure is 

 accomplished by the intercalai'ia (fig. 296 and fig. 297, 

 partly between le). < h- the covering may consist simply 

 of a fibrous niembi'ane. The neural arches sometimes 

 (e. g. in the Rays) grow downwards along the side's of 

 the vertebne or even to their ventral side; but as a 

 rule htcmal arches (figs. 296 and 297, ha), lower inter- 

 calaria (ih). and lower tei'minal parts (spinous ))roeess- 

 es) are developed in a downward direction, their extent 

 and alterations being similar to those we have just re- 

 marked in tlie eont'Spondiiig parts of the neural canal. 

 In the anterior jiart of the bod\-, behind the head, both 

 the vertebra.' and tiieir a])opiiyses often coalesce into a 

 continuous mass of cartilage, piei'ced only by the spinal 

 cord and the spinal nerves; and in this part ribs are 

 wanting or re])laced by a continuous fibrous membrane, 

 whereas costal cartilages are present further back. Such 

 is the case in the Hays. In the Sharks, on the other 

 hand, the vertebra' are more regularly differentiated 

 throughout the spinal column; and ribs, though some- 

 times wanting, are more frecpiently present throughout 

 the abdominal region. The Ifnhvrphnl) (fig. 294) show 



" Poiss. Fo-fn., toni. Ill, pp. 1 .and 212. 



