KINSHIPS OF CHIM^ROIDS 



113 



sense organs : they present similar visceral characters, 

 spiral intestine, heart, gills, abdominal pores, renal and 

 reproductive organs. 



Their more important divergences from the plan of 

 elasmobranchian structure may thus be summarized: — 



I. Skull and mandible (v. pp. 252, 256). The mandi- 

 ble articulates directly with what appears to be the carti- 

 lage of the cranium, i.e. without the hyoid-arch element 

 serving as the suspensorium {Autostylic, p. 257). 



II. Fins, paired (Wiedersheim) and unpaired (Ryder), 

 and fin defences. The first dorsal, armed with an anterior 

 spine, is so specialized that it folds like a fan, and may be 

 depressed into a receptive sheath. The tail is (second- 

 arily) diphycercal. 



III. Skin defences and teeth. Shagreen tubercles 

 occur in Chimaeroids and are in every way shark-like. 

 They are scattered thickly over the entire dorsal region 

 in Menaspis* sparsely in Squaloraja. They occur in the 

 head region and on the spines in Myriacanthus (Figs. 106 

 C, 114); and on the head, spine, and clasper tips of recent 

 forms (Figs. 113 D, i\6 D). But dermal bones also occur, 

 as in Myriacanthus (Fig. 106 B), which do not outwardly 

 resemble the structures of ancient sharks shown, e.g. in 

 Fig. 90 B. The dermal plates protecting the suborbital 

 sensory canal of Chimaera (Fig. 104, DP) must be looked 

 upon as specialized defences, not as degenerate remnants 

 of a complete dermal armouring (Pollard). And the dental 

 plates, as already noted (p. 99), are altogether unshark-like ; 

 their tritors are few in number and constant in position, 

 suggesting an origin from more superficial tooth centres, 

 but these in turn, like the toothplates of Cestracionts, may 

 have been evolved from shagreen denticles. 



* Jaekel, SB. d. Gesell. nat. Freunde, Berlin, 1891, Nr. 7. 

 I 



