246 CALIFORNIA ACADEMY OF SCIENCES. 



184. Doras nauticus. 



Zathorax nauticus Cope, Proc. Acad. Nat. Sci. Phila. 1874, 133 



(Nauta); id. Proc. Am. Philos. Soc. xvii, 678, 1878 (Nauta). 

 Habitat: Marafion. 



This species is known only from the types. 



XLI. OXYDORAS. 



Oxydoras Kner, SB. Ak. Wien, xvii, 115, 1855 (sp.) 



Oxydoras Bleeker, Nederl. Tijdschr. Dierk. i, 14, 1863 

 (niger). 



Pseudodoras Bleeker, Nederl. Tijdschr. Dierk. i, 14, 

 1863 (d'Orbigny). 



Rhinodoras Giinther, v, 209 (sp.) 



Type: Doras niger Valenciennes. 



Dr. Bleeker departs from his usual rule of considering^ 

 the first species the type of the genus, and establishes 

 niger as the type of Oxydoras. As this is the first restric- 

 tion of Oxydoras to any type it should be adopted. 



This genus is most readily distinguished from Doras 

 by the posterior position of the eyes. Maxillary barbels 

 simple. Snout long and pointed. Adipose fin long and 

 low, gradually merging into the dorsal outline anteriorly. 

 Dorsal spine serrate on both margins. Sides with a series 

 of hook-bearing plates. 



Habitat: Rio Plata; Amazon and Guiana region. 



ANALYSIS OF THE SPKCIES OF OXYDORAS. 



a. Teeth none; posterior nostril nearer the anterior nostril than to the 

 eye; anterior nostril remote from tip of snout; eye situated wholly in 

 the posterior half of the head; dorsal spine with much stronger teeth 

 in front than behind; gill-openings continued below to a point directly 

 behind the ricti;s; head mi;ch longer than broad. (Oxydoras.) 



b. Lateral scutes 17-25. niger 185. 



bb. Lateral scutes 34. knerii 186. 



aa. Teeth present; posterior nostril nearer the eye than to the anterior 

 nostril which is close behind the lip; eye equidistant between tip of 

 snout and posterior margin of opercle, or slightly nearer the latter;, 

 dorsal spine with much weaker teeth in front than behind; gill-openings 

 not continued below the pectoral spine; head about as long as broad. 

 D. I, 6; A, 12. (Ehinodoras.) 



