76 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



Pacific Oceans. Mr. Darwin's specimen agrees in every respect with the descrip- 

 tion in the "Histoire des Poissons," except in having one ray more in the anal fin : 

 its length is not quite five inches. 



2. AcANTHURUS HUMERALIS. CuV. et Val. 



Acanthurus humeralis, Cuv. et Val. Hist, des Poiss. torn. x. p. 170. 



Fohm. — General form oblong-oval. Greatest depth just behind the insertions of the pectorals; 

 contained exactly twice in the length of the oval of the body (measuring this last from the end 

 of the snout to the base of the caudal spine), and three times in the entire length (measuring 

 this last to the extremities of the lobes of the caudal fin.) Profile convex before the eyes, 

 whence it descends nearly vertically to the mouth. Height of the head a little exceeding its 

 own length. Eyes very high in the cheeks, and in front of each a grooved line passing hori- 

 zontally forwards towards the nostrils; which last consist of two small round orifices, the 

 anterior one larger than the other, and partially covered by a membranous flap. There are 

 seventeen teeth in the upper jaw, and sixteen in the lower : those above have the cutting edges 

 crenated, and likewise the lateral edges for nearly halfway down ; this most observable in the 

 middle ones, in which the crenations amount to eight or ten in number: those below similar, 

 but with the crenations not quite so numerous, and in some of the teeth at the sides of the jaw 

 almost confined to the cutting edges. Scales minute ; those taken from the middle of the body 

 appear of an oblong form, their apical portions dotted, and ciliated with from twelve to eighteen 

 very minute denticles, their surface marked with extremely fine delicate stria;, not distinguish- 

 able without a strong lens. 



The lateral line follows the curvature of the back, at about one-fifth of the depth. The spine 

 on the sides of the tail is strong, and sharp-pointed, and very slightly bent. No reclined spine 

 before the dorsal. Both the fin just mentioned and the anal have their soft portions terminating 

 posteriorly in rather an acute angle : also both have a scaly membrane at the base, and rows 

 of minute scales between the soft rays extending for about one-third or more of their length. 

 The first ray of the anal is very minute, and so much concealed in the skin as to be easily 

 overlooked. The filaments of the caudal are sharp-pointed, and extend as far again as the 

 middle rays : the upper one rather longer than the other. The pectorals are contained three 

 times in the length of the oval of the body. Ventrals attached a little further back, sharp- 

 pointed, and terminating in the same vertical line with the pectorals, both being laid back. 



D. 9/23 ; A. 3/23 ; C. 16, &c. ; P. 16 ; V. 1/5. 



Length, to the end of the caudal lobes, 7 inches. 

 Colour. — The colours appear to have been exactly as described in the " Histoire des Poissons." 

 Mr. Darwin's notes taken from the recent fish state, "splendid verditer blue and green;'' but do 

 not enter into the details of the markings. 



Habitat, Tahiti. 



Obtained at Tahiti, where it had been previously found by MM. Lesson 

 and Garnot. Mr. Darwin's specimen accords with the characters given by Cuvier 



