2 1 6 edwin chapin starks. 



Relationships. 



Dr. Gill in the " Standard Natural History " ' places the family 

 Siganidae with the Acanthuridae under the superfamily Teuthido- 

 idea. The Teuthidoidea he believes to be descended from the 

 Chaetodontoid fishes while the plectognathous fishes are descended 

 from the Teuthidoidea. 



Dr. Jordan in his " Guide to the Study of Fishes " 2 follows 

 the same arrangement but he places the families Acanthuridae 

 and Siganidae together with the Chaetodontidae and other related 

 forms in a large group, the Squamipinnes, though he considers the 

 Acanthuridae and the Siganidae under different suborders, giving 

 to the latter suborder the name Amphacanthi. 



Either of these, with slight changes, is the order in which 

 these fishes are arranged by all modern authors, and as it is ap- 

 parently the most logical no other arrangement need be here 

 referred to. 



Though there is doubtless an alliance between the families 

 Acanthuridae and Siganidae the alliance is certainly not close 

 enough to place them in the same superfamily. The Teuthido- 

 idea is defined by the " development of transverse, expanded, 

 buckler-like, subcutaneous plates on the back intervening between 

 the spines, and limiting their erection forward." 



The expanded interspinous rays that form the bony bucklers 

 at the base of the spines are developed in this respect only to a 

 slightly greater degree than may be found in many scombroid 

 fishes, and not to so great a degree as in some berycoid fishes. 



The fishes of the Acanthuridae 3 differ from those of the Sigan- 

 idae in having the cranium wedge-shaped or tapering to a point 

 at the ethmoid region, the parasphenoid drawn out downward 

 into a wide, thin, sharp plate before the orbital region ; the post- 

 orbital part of the cranium shortened ; the parietal present ; the 

 post-temporal suturally attached to the cranium and forming an 

 integral part of it (this true of Hepatus to a greater extent than of 



1 Cassino& Co., Boston, 1885, Vol. III. 



2 Henry Holt & Co., New York, 1905, Vol. II. 



3 Hepatus bahianus and Xesurus punctatus represent the family Acanthuridae in 

 this investigation. 



