334 ACIPENSERID^. 



species. Ho also states that hybrids arc not unfrequeiit, especially 

 between A. stellatus and A. ruthcnux, and A. hvso and A. schypa 

 (Bull. Soc. Nat. Mosc. 1857, ii. pp. 530-545). 



The species have been referred to several subgenera, some of which 

 were characterized by the form of the keel on the dorsal shields. In 

 some the highest point of the keel is at its extremity (0/;>?'s^/(oc(;«^rfs), 

 in others before the extremity, nearer to its middle (Mesocentres). 

 It is true that certain species are " Opisthocentres " throughout life ; 

 but a number of others which are *' Mesocentres " in mature age are 

 ". Opisthocentres " when young, and it is therefore better to distin- 

 guish the former by a more reliable character, namely, by the in- 

 creased number of lateral shields. 



Although only a few of the species recently distinguished by Aug. 

 Dumeril are admitted here as such, I am afraid several of them will 

 prove to be merely nominal species also. 



Lat. shields GO-70. Snout pointed, narrow . . I. ruthenm, -p. 335. 



Lat. shields 60. Snout rather short, rounded ; the halves of the lower lip 

 contiguous 2. (/laber, p. o3o. 



Lat. shields 49-51. Snout rather pointed ; the halves of the lower lip 

 separated by an interspace 3. brandtii, p. 336. 



Lat shields 42-47. Snout with small osseous scutes; parietal and tem- 

 poral plates subequal in size 4. transmontanus, p. 3;!(i. 



Lat. shields 40-42. Snout with small osseous scutes ; parietal plates 

 conspicuously larger than the temporal ; the first caudal fulcrum very 

 small 5. naccarii, p. 336. 



Lat. shields 36. The first upper caudal fulcrum much larger than the 

 following 6). hrachyrhynchu^, p. 337. 



Lat. shields 41. Snout with a large osseous shield above. 



7. nasus, p. 337. 

 Lat. shields 40-45. Snout without osseous scutes above. 



8. huso, p. 337. 

 Lat. shields 40-41, dorsal 17. Skin entirely naked. 



9. sinensis, p. 338. 

 Lat. shields 34-35, none along the abdomen. 10. mhictoidus, p. 338. 

 Lat. shields 33-38. Snout pointed, nearly one-half of the head *. 



11. macnlosus, p. 339. 



Lat. shields 30-35. Snout very long and narrow, nearly two-thirds of 

 the head 12. stellatuf, p. 340. 



Lat. shields 20-.32. Snout short, obtuse, about one-third of the head. 

 Shields coarsely radiated 13. f/ii/dciisfadfii, p. .340. 



Lat. shields 33. Snout rather short and pointed. Sliidds finely sculp- 

 tured and rather smooth 14. Uopdti^, p. 341. 



Ijat. shields 22-28. Snout very short and obtuse, about one-fourth of the 

 head. Shields finely radiated 15. hrevirodris, p. 341. 



Lat. shields 28, dorsal 11. Anal entirely behind the dorsal. 



16. mediorostris, p. .342. 



Lat. shields 26-31, dorsal 11-13. Snout pointed, about one-half of the 

 head 17. stnrio, p. 342. 



Lat. shields 28-29, dorsal 9. Snout one-half of the liead. 



18. ayassizii, p. 344. 



Lat. shields 27-29, dorsal 9-11. Snout longer than the remaining part of 

 the hi'ad 19. (ic>tf.inisf/i>:. p. 344. 



* Moasured to tho end of the operculum. 



