SUB -CLASS I 



SELACHII 



17 



of the Leptocardii (Amphioxus) and Cyclostomi (lampreys and hag-fishes) as 



independent classes equivalent to that of the fishes, 



and in the closer union of the ganoids and bony fishes, 



which are frequently grouped under the common 



denomination of Teleostomi. The class of fishes in its 



restricted sense thus comprises the four still-existing 



sub-classes of Selachii, Dipnoi, Ganoidei, and Teleostei ; 



while to these must be added the extinct sub-classes 



of Ostracodermi (or Ostracophori) and Arthrodira, which 



are restricted to the Palaeozoic. 



No fossil remains of Leptocardii are known. It 

 must also be remarked that there is no satisfactory 

 evidence of extinct representatives of the Cyclostomi 

 or Marsipobranchii. Certain annelid jaws from the 

 Palaeozoic formations, known as Conodonts, were once 

 erroneously supposed to be teeth of this class (see vol. i. 

 p. 255). The problematical organism from the Old 

 Red Sandstone of Achanarras, Caithness, named Palaeo- 

 spondylus gunni 1 by Traquair (Fig. 30), is likewise very 

 doubtfully placed here. The largest specimens of this 

 form do not exceed 0'05 m. in length. The skull 

 terminates in front in a ring of calcified cirri (d.c, l.c, 

 v.c), which surround a large bilaterally symmetrical 

 orifice {it). Behind the skull there extends backwards 

 a pair of elongated cartilages (x), usually lying in the 

 fossil parallel with the vertebral column. There are 

 vertebral centra in the form of broad rings • in the 



these bear short and stout neural 



Fig. 30. 



abdominal region 



Palaeospondylus gunni, Tra- 

 quair. Lower Old Red Sand- 

 stone ; Caithness, a, Separate 

 small lobe ; b, Anterior depres- 

 sion or fenestra ; c, Posterior de- 

 pression or fenestra ; d.c, Dorsal 



spines, out no rios ; in tne caudal region tne neural cirri ; z.c, Lateral cirri ; %,Suj- 



1 ' J o posed narial opening ; p.a, rara- 



and haemal spines are very slender, while the former cnordai portion of skull; t.p, 



t ,-, -11 i t •• .i t i , • Trabecule - palatine portion of 



are longer than the latter and distinctly dichotomise skull; v.c, Ventral cirri ; *,Pro- 

 at the distal end. There is no undoubted evidence of §^STnS£S iBitilBi * te8; 

 paired limbs. 



but no ribs ; in the caudal region the neural 



Sub-Class 1. SELACHII. Cuvier. 



(Elasmobranchii, Bona]).; Chondropterygii, Cunther; Placoidei, Ag.) 



Endoskeleton cartilaginous; exoskeleton, when present, structurally identical 

 with the teeth (placoid scales). Usually five {rarely six or seven) pairs of branchial 

 arches, the clefts exposed or covered only hy a flap of skin, without operculum. In 

 the living forms — optic nerves not decussating but forming a chiasma; bulbus 

 arteriosus of the heart with two, three or more series of valves; air-bladder absent ; 

 ■intestine with a spiral valve ; ovaries with feiu large ova. 



The unsegmented cartilaginous cranium of the Selachii is more or less 

 imperfectly hardened with polygonal granules of phosphate of lime ; but the 

 vertebrae are very variously calcified, can thus be readily preserved in a fossil 

 state, and, according to Hasse, display features of special importance for the 



1 Dean, B., The Devonian "Lamprey" Palaeospondylus gunni, Traquair (.Mem. N.Y. Acad. Sci. 

 vol. II. pt. 1), 1900. Bibliography. — Traquair, A'. //.. A still further contribution to our knowledge 

 of Palaeospondylus (Proc. Ptoy. Phys. Soc. Edinb. vol. XII.), 1894. 



VOL. II C 



