THE ORDERS OP TELEOSTOMOUS FISHES 497 



Superorder Acanthopteroidei {cont'd). 

 (Plate XXIX.) 

 Order Percesoces 1 Cope. 



This group, being on the borderland between soft-rayed and 

 spiny-finned fishes, may be classified with either, according to 

 the characters selected to separate the spiny-finned fishes from 

 the orders that lead up to them. Cope proposed the term to in- 

 clude only the Atherinidae (Silversides), the Mugilidae (Mullets), 

 the Sphyraenidae (Barracudas), but Smith Woodward and 

 Boulenger 2 include not only the Synentognathi but also several 

 families which are regarded by the American school as Acan- 

 thopterygii. 



If we accept the term in its limited sense the order is readily 

 defined from the Synentognathi, on the one hand, and from 

 the true Acanthopterygii, on the other. Although certain 

 Percesoces (e.g. Atherina araa) show a general resemblance to 

 the more generalized Synentognathi (e.g. Chriodorus atherin- 

 oides), and although fossil forms may be discovered, intermediate 

 between the two orders, yet in the Percesoces a trenchant dis- 

 tinction from the Synentognaths and other physoclists with 

 abdominal ventrals is afforded by the fins, in the appearance (i) 

 of a separate spinous dorsal more or less remote from the soft 

 dorsal, and (2) of an anterior spine in the pelvic fins. Further- 

 more the ventral fins are more forward than in the Synentog- 

 nathi and the pelvic bones (save in Sphyraenidae) are either 

 attached to the backwardly produced postclavicles (Mugilidae, 

 Polynemidae) or by ligament to the clavicular symphysis (Ather- 

 inidae, Chiasmodontidse) . The ventral fin formula is now re- 

 duced to I, 5. In order to differentiate the Percesoces from the 

 true Acanthopterygii ' ' we must turn to the well known external 

 -characters— a spinous dorsal in conjunction with the abdominal 

 ventral fins, high pectoral fins, and unarmed opercles [i.e. opercu- 

 lum and preoperculum without posterior spiny processes]." 

 (Starks. 3 ) 



1 Perca, perch, esox, pike, in allusion to the mingling of ancathopterous 

 and haplomous characters. 



2 Cambr. Nat. Hist., Vol. "Fishes," etc., p. 636. 



3 Starks, E. C, "The Osteological Characters of the Fishes of the Sub- 

 order Percesoces," Proc. U. S. Nat. Mus., Vol. XXII, 1899, pp. 1 etseq. 



