16 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



of the pectorals, which appears to be a character of some importance, and which 

 has led to the generic name of Aplodactylus, amounts in this specimen to six, 

 being two more than was observed by M. Valenciennes in his, though the total 

 number of rays in this fin is the same. I may also allude to the circumstance of 

 the dorsal being invested at the base with a thickened membrane on each side, 

 closely covered with small scales, which extends over nearly its whole length, 

 but is most conspicuous along the spinous portion. This character is not men- 

 tioned by M. Valenciennes. Neither does he mention the rows of minute scales, 

 which occur between the rays of all the fins, except the ventrals. 



Mr. Darwin's specimen of this fish is eleven inches in length. The following 

 is the fin-ray formula : 



B. 6; D. 15— J/21 ; A. 3/8; C. 17. &c. ; P. 9— vi ; V. 1/5. 



1. Dules Auriga. Cuv. et Vol. 



Dules Auriga, Cuv. et Vol. Hist, des Poiss. torn. iii. p. 83. pi. 51. 



Form. — This species is remarkable for the prolongation of the third dorsal spine, which, in the 

 present specimen, is not quite equal to half the entire length of the head and body ; a small 

 portion, however, appears to have been broken off. The greatest depth is contained three and 

 a half times in the entire length. The head, measured to the extremity of the opercular 

 membrane, exactly equals the depth. The line of the profile is not quite straight, there being 

 a slight depression at the nape, above which is a convexity in immediate advance of the dorsal 

 fin. The lower jaw is a very little the longest. The eyes are large ; and the distance between 

 them barely equals their diameter. The other characters are exactly as stated in the " Histoire 

 des Poissons." 



B. 6 ; D. 10/13; A. 3/7 ; C. 17 ; P. 17 ; V. 1/5. 



Length 5 inches 3 lines. 



Colour. — The recent colours are given by Mr. Darwin in his notes as follows : " Sides with numerous 

 waving longitudinal lines of brownish red ; the intermediate spaces greenish-silvery, so figured 

 as to look mottled. Head marked with lines of dull red and green. Ventral and anal fins dark 

 greenish blue." — He does not notice the vertical bands alluded to by Cuvier and Valenciennes, 

 which are sufficiently obvious, and which accord with the figure and description of the authors 

 just mentioned. 



Habitat, Maldonado Bay, Rio Plata. 



