42 ZOOTOMY. 



apertures of the sensory tubes : the only large spines are on the tail : 

 the greater part of the skin is smooth. 



In R. clavata, the skin is brown above, white below : the dorsal 

 surface of the whole body is covered with asperities often produced into 

 distinct spines : the latter are especially large near the eyes and in the 

 middle line of the back and tail : in the female especially, large spines 

 occur on both dorsal and ventral surfaces of the body. 



In R. maculata the dorsal surface is brown with darker brown or 

 black spots, the ventral surface white : the skin is nearly smooth, 

 except on the tail and in the region of the eyes : in the male there is a 

 patch of large movable spines on each side of the head and on each 

 pectoral fin, both on the dorsal surface. 



6 1. The mouth, a wide, transverse aperture on the 

 under surface of the head, supported by the strong upper 

 and lower jaws, which can be felt through the skin. 



62. The teeth, forming a close pavement of small en- 

 amelled plates, produced in some cases into sharp spines. 



In R. clavata the teeth are pointed in the male, flat in the female. 



63. The nostrils, moderate-sized apertures, one on either 

 side of the under surface of the head, a little in front of 

 the mouth. 



64. The fronto- nasal process, an escutcheon-shaped 

 fold of integument, extending from the posterior boundary 

 of tl^ nostrils backwards towards the mouth, at either angle 

 of which it forms a rounded lobe, beset with fimbriae. This 

 lobe bounds internally, and converts into an incomplete 

 canal, a groove the naso-buccal groove which ex- 

 tends from the nostril to the angle of the mouth. 



65. The branchial apertures, five valvular slits on the 

 ventral surface of the body, behind the mouth, those of 

 opposite sides approaching one another posteriorly. 1 



1 Behind the last pair of branchial apertures, on the skin covering the 

 ventral portion of the shoulder girdle, is a pair of slit-like depressions, 

 looking like an obliterated sixth pair of gill slits. 



