KINSHIPS OF CHIMAEROIDS 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 (Vv. pp. 252, 256). The mandi- 
ble articulates directly with what appears to be the carti- 
lage of the cranium, ze. without the hyoid-arch element 
serving as the suspensorium (Awfostylic, 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 Chimeroids 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,116 D). But dermal bones also occur, 
4 asin Myriacanthus (Fig. 106 4), 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 Chimera (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. 
* Jackel, SB. d. Gesell. nat. Freunde, Berlin, 1891, Nr. 7. 
