302 Mr. G. A. Boulenger on the 



D. S. Jordan regarded a liigli number of vertebrse, otlier 

 things being equal, as indicative of generalization, and even 

 thought the Pleuronectidai afforded support to this view. 

 Not aware of the state of things in the Hippoglossine 

 Pseltodes, he wrote * : — " Thus in the comparatively primi- 

 tive subfamily of Hippoglossinge, the halibut group, the 

 division nearest the cod-like stock from which the flounders 

 are probably descended, the numbers range from 35 to 50. 

 In the turbot group (Psettinffi) from 31 to 43." I have 

 gradually arrived at the conclusion that Jordan's theory 

 cannot be applied to the various groups which make up the 

 suborder Acanthopterygii, and that the explanation of the 

 fact that so many of its marine members agree in having 24= 

 vertebrae is due to common descen^Vom a Cretaceous marine 

 type, probably Berycid, in which tne number had been thus 

 reduced. Further evolution would again have tended to an 

 increase of the segments, especially in freshwater, deep-sea, 

 and pelagic forms, for physiological requirements, which, 

 liowever, are not always clearly apparent. The " natural 

 selection " theory, by which Jordan has endeavoured to 

 explain the variability in the number of vertebrae within 

 restricted groups, can have no further claim than that of 

 ingeniousness, since it implies a reversion of the evolution- 

 lines that can be followed in the minor groups of the Pleuro- 

 nectida?, especially the Hippoglossina? and Soleinse. 



Another good reason for regarding the Amphistiidse and 

 Zeidge as related to the ancestral type of the Pleuronectidai is 

 the fact that the ventral fin of the latter, although always 

 much reduced, contains frequently as many as six articulated 

 rays, sometimes with the addition of a simple ray {Lii2?po- 



US) 



I therefore propose the establishment of a division of the 

 suborder Acanthopterygii, under the name of Zeorhombi, to 

 be defined as aberrant, strongly compressed Perciformes, with 

 very short prgecaudal region, modified in the direction of the 

 flat-fishes, and characterized by the combination of an 

 increased number (7 to 9) of ventral rays, with absence of 

 hypural spine (by which Berycidaj are excluded), or by 

 asymmetry of the skull in the forms in which the spine of 

 the ventral fin has been lost. 



This division embraces tliree families only : — 



* ' Temperature and Vertebrae : a Study iu Evolution ' (Ithaca, 1893), 

 p. 25. 



