150 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



PLECTOGNATHI. 

 Family.— TETRODONTID^. 



1. DiODON NYCTHEMERUS. CuV. 



Diodon nycthemerus, Cuv. Mem. du Mus. torn. iv. p. 135. pi. 7. 



A species of Diodon in Mr. Darwin's collection, the number attached to 

 which has been lost, and of which the locality is in consequence unknown, appears 

 referable to the D. nycthemerus of Cuvier. 



The spines are long, measuring three quarters of an inch in length ; round, sharp, and not 

 very close together. There are five in the front row between the eyes, seven in a transverse 

 row between the pectorals, and ten or eleven between the snout and the dorsal in a longitudi- 

 nal one : none exactly on the upper part of the tail, but one on each side of the base of it, a 

 little below the termination of the dorsal fin, and a corresponding pair still lower down. The 

 spines on the belly are shorter, and rather closer together than those on the back. One of 

 those on the back in this specimen is accidentally forked. 



The true teeth appear on the surface of the jaws like minute scales, as in several species 

 of the genus Scarus. 



The fin-ray formula is as follows : 



D. 13; A. 13; C. 9 ; P. 20. 



Length 5 inches 6 lines. 

 The colours, so far as can be judged, the specimen being in spirits and not in 

 very good condition, answer to Cuvier's description of them with tolerable exactness. 



2. Diodon rivulatus. Cuv. 



Diodon rivulatus, Cuv. Mem. du Mus. torn. iv. p. 129. pi. 6. 



An individual apparently of this species was picked up by Mr. Darwin on the 

 shore of the Rio Plata at Maldonado. It agrees with Cuvier's description, except- 

 ing that the undulating lines are not visible, probably owing to the state of the 

 specimen when found. 



The spines are short, barely a quarter of an inch in length, but very strong, compressed, 

 and resembling canine teeth. There are three in the first row between the eyes; about six in 

 a transverse row across the back, and seven or eight in a longitudinal one. Beneath they are 

 shorter and more numerous. The orbits are elevated in ridges, and project forwards over the 

 eyes. Two very small barbules attached to the lower lip. Surface of the jaws smooth, the 

 teeth not appearing as scales. 



D. 11 ; A. 10; C. 8 ; P. 22. 



Length 5 inc. 3 lin. 



