1078 



SCAXDIXAVIAN IISHES. 



ELASMOBRANCHII HOLOCEPHALI'. 



Elasmobranchs with one gill-opening- common to all the gill-slits on each side of the body. No spiracles. Skin 



smooth, without placoid scales (spines'). Notochord unaltered, not constricted by the formation of vertebrse, 



only with superficial rings of cartilage, considerably exceeding in number the vertebral spaces (polyspondyli). 



Palatoquadrate part not divided from the skull, but furnished like the lower jaw with dental disks. 



That course of (k'velopioent fruui the primordial j cartihiges admit of interpretation as vestiges of aborted 

 fishes' wliieli is represented in the jiresent age by the spiracular gills. On the other linnd, tiie structure of 



HoJnccphali, the suborder of tlie Chinueras, has been 

 arrested as regards the structure of the spinal column 

 at a lower stage than the Plagiostoms. It is probable 

 too that the structure of the palate in the Holocephali 

 is a relic of the .said ancestral types; Init only the 

 iiistory of evolution can decide whether the union lie- 

 tween the palatoquadrate parts and the skull, a pecu- 

 liarity which we shall again meet with in the still 

 more primitive Marsipobranelis, is here of a primary 



the l)ranchial cavity in tlie Ilolocejihdii more clearly 

 indicates a higher degree of differentiation, a .step in 

 the direction of the Chondrosteans and Teleosts. 



Tlie HoJdcepliali are marine and deep-sea fishes 

 with a wide geogra]jhical I'aiige. How great is their 

 geological age, can hardly he stated at present with 

 certainty, for ichthyologists have sujjposed'' that remains 

 of these fishes have been discovered even in Devonian 

 deposits. It is certain, however, that tliey lived in the 



or a secondary nature. Equally indispensable is the aid | Jurassic period, when its very earliest stratum (the 

 of the history of evolution to a determination whether Lias) was formed; and during the latter part of this 



the absence of spiracles is original or rather the con- 

 sequence of reduction. Solger has described in Chi- 

 meera monsfrosa small cartilages (iig. 294, .s) which 

 occupy in relation to tlie lower jaw the same position 

 as that assumed in relation to the jialatoquadrate car- 

 tilage by tlie spiracular cartilages (supports of the spi- 

 racular gills) common in tlie Plagiostoms. It may 

 reasonably be assumed tiiat tliese so-called Solgerian 



period the)' attained a size far surpassing tiiat of mo- 

 dern Holneephali. Townsend, for instance, found in 

 the Portland ciialk at Great Milton (near Oxford) an 

 under tooth of Chima'.ra {Ischyodon) Toirnsendn' that 

 measured 1 1 cm. at the symphyseal margin, a dimen- 

 sion whicli in a Chimcera moiistrasii If) cm. long is 

 re|)resented by 14 mm. 



The suborder contains only one family. 



I AM CHIMTERIDiE. 



liodij (if a compressed rlurute form^ most nearly resernhVuKj that of the Macniroids. Dentition of tlie 7iioidh made 

 up of tiro upper pairs and one inider pair of dental plates, most similar to those of the Lan/i-fishes {Dipnoi). 

 Tw<r' dorsal fins, the anterior, (diore the pectoral fins, triangidar and armed irith spines, the posterior loir ami 

 long: a small, sometimes scarcely distinyiiishahle anal fin, situated far haeh: caadal fin dijihycereid or lieterocercnl. 



Of this family only four'' s})ecies survive at the pre- 

 sent time. These have been ranged in two genera, one of 



which, the Antai'ctic Callorhynchas, contains a solitary 

 species and is characteiised by the more obliquely for- 



" olog, wliole, and y.iC(ia7,l\, head. 



' In the ycmng- of Callorhijnclius (a genus frnm tlie Pacitic Ocean anil the Antarctic regions) spines liave been found in two lougilu- 

 dinal rows, |iarlly on the forehead, partly on the back, between the first and second dorsal fins (see DiJJi., Hist. Nat. Poiss.. Snites a Biiffon, 

 tonic I, p. 094, pi. 14, fig. 4), an indication that the skin of the primitive forms was clothed with spines. 



' Pisces aspondyli, Basse, Natiirl. 6'i/si. Elasmohr., Allgcni. Tlieil, ]>. 31. 



'' Of. GtJNTHEK, Introd. Study Fish., p. 349. 



' AOASSiz, Rech. Poiss. Foss., tome III, p. 343, tab. 40, Iig. 20. 



f Sometimes the posterior dorsal fin is so deeply sinuons that tlircc dorsal fins have lieen counted. 



" Gill has besides described a Cliinurra pluinhfii from the Atlantic consi of North America; but there would hardly seem to he any 

 specific distinction between this form and our (Jliima'ra. 



