ACANTHIDIUM PUSILLUM. 41 



genus. The position of the second dorsal fin is, speaking strictly, interme- 

 diate between that which obtains in either of these genera. It is backwarder 

 than in Spinax, Cuv., but rather forwarder than in Centrina, Cuv., whilst, 

 in its large size compared with the first dorsal fin, it disagrees with both. 

 The caudal fin resembles that of Sjjinax rather than Centrina. 



The constancy of these distinctions has now been verified repeatedly 

 in several individuals of this little Shark : and their value in a gene- 

 ric point of view may be considered incidentally and independently con- 

 firmed by the proposal of the genus Spinax, Buon., upon similar if not the 

 self-same characters, and by its adoption by two authors, who have devoted 

 so much labour and talent to the study of this branch of Ichthyology as 

 MM. Muller and Henle. 



The little Shark here figured is very imperfectly known to the fisher- 

 men, and must be accounted of excessive rarity. Five or six individuals 

 only have occurred, of which all but one were females. In size, these 

 exhibited a remarkable uniformity : and the fishermen affirm positively, 

 that this Shark does not exceed a foot in length. 



The general form is elongated and slender, especially towards the tail : the 

 muzzle, head, and nape are flattened above, but have a swollen, vaulted, or inflated 

 appearance : the body is subprismatic, depressed forwards, slightly convex along 

 the top of the back as far as to the second dorsal fin, the belly forming the broad 

 side of the prism : behind the second dorsal fin the back is faintly channeled : 

 muzzle broad and thick, subtrilobed or triangular, contracting suddenly imme- 

 diately before the nostrils, just behind which it is broadest, and produced into 

 an intermediate narrower, obtuse, short, thick, terminal lobe : the nostrils being at 

 the anterior face, or partly on the under side, of the two much less distinct and 

 shorter side lobes, and situated rather less than half-way from the tip of the middle 

 lobe to the eye ; they are large, distinct, and double or bilobed. Their upper 

 valve is narrow and acuminate ; their lower or hinder shorter and triangular. The 

 breadth of the muzzle just behind the nostrils exceeds considerably its length from 

 the fore-comer of the eyes. Eyes very large, oval, with the orbits extending both 

 forwards and backwards into corners considerably beyond the eyeball ; the hinder 

 canthus is in a vertical line with the mouth, which is situated quite underneath. 

 They have no nictitating membrane. The spiracles are large and conspicuous, 

 semilunar, or half oval ; and with an internal, dark-coloured, valvular dissepiment; 

 they are placed on the upper side of the neck or nape, about half the longitudinal 

 diameter of their orbit behind the inner canthus of the eyes, and half-way between 

 the tip of the muzzle and the anterior base or origin of the pectoral fins. Branchial 

 slits or openings five : the first is about as far behind the spiracles as these are 

 behind the eye ; the last is close before the front base of the pectoral fin, partly 

 below and partly above its level. Mouth semilunar, strongly plaited or wrinkled 

 at the comers, with a smooth sort of narrow underlip. Teeth different in the two 

 jaws ; in the upper are three rows in front, two on the sides, of small, sharp-pointed^ 

 straight, triangular-acuminate or thorn-shaped teeth ; their points directed back- 

 wards, and having at their base a small equal toothlet or two on each side ; they are 

 placed behind each other, not quincuncially : in the lower jaw there is only a 

 single compact and even row of about forty incisorial somewhat square teeth, the 

 short points of which are abruptly deflexed horizontally, diverging from the centre 



