26 



SORTAT1ON. 



carry the rays of the caudal fin, so that the homocercal tail is only 

 superficially symmetrical after all. Compare this with the other 

 type sketched in Fig. 10, in which the backbone is continued to the 

 outermost point. 



In our specimen fish the lobes of the tail are equal, and there is 

 nothing approaching a heterocercal character ; we will, therefore, 

 sort out our heterocercals forthwith. One genus is so distinct that 

 it claims first place. In our waters it is represented by a single 

 species, which is the sole representative of the family and the sub- 

 order. This is the sturgeon, Acipenser sturio, recognisable at once 

 by the five rows of bony plates, the long snout, the small dorsal fin 

 placed near the tail, and last, not least, when it comes to table, the 

 notochordal backbone. Externally the plates distinguish it at a 

 glance ; of one of them, which hang side by side like a long row of 

 shields, we give a sketch (Fig. u). 



With the sturgeon out of the way, we have left on our hands the 

 fifteen British sharks, which will not prove troublesome. Four of 

 them have no anal fin, and of these four two have spines in their 

 dorsals and two have not. Of the two with spiny dorsals, one, 



Centrina, has the spine starting 

 from the middle of the base of the 

 fin and curving forwards across it, 

 as if it were a sprit used to keep 

 the fin extended ; the other, 

 Acanthias, has the spine in the 

 front of the dorsal running up the 

 fore-edge in the usual way. Of 

 Centrina only one example (C. 

 salviani) has been caught in British 

 waters, and consequently our 

 sharks with no anal fin and 

 spiny dorsals are practically the 

 common spur dog-fish, A. vulgaris. 



In the next two genera the dorsals are without spines, but the 

 skin is spiny. In one, Lczmargus, the first dorsal is some distance 

 in front of the ventrals ; in the other, Echinorhinus, it is far back 

 and over them. The former is represented by L. microcephala, the 

 Greenland shark, a bulky species with a prominent snout and a 

 sort of upper lip ; and of the latter the only representative is 

 E. spinosus, the spinous shark, in which the snout is of the normal 

 conical shape. Here, then, is our first category of sharks in 

 tabular form : 



Anal absent 



Dorsals with spines 



Spine in the middle of each dorsal Centrina. 



Spine in front of each dorsal Acanthias. 

 Dorsals without spines 



First dorsal in middle of back Lamargus. 



Both dorsals near tail Echinorhinus. 



Fig. ii. SHIELD OF STURGEON. 



The sharks of our waters that have an anal fin are eleven in 

 number, and with a single exception they all have two dorsal 



