THE ORDERS OF TELEOSTOMOUS FISHES 501 



resemble Mene rather closely. The supposed relationships 

 either to the Thoracosjraci or to the Scombriformes turn upon 

 the resemblances of the shoulder girdle to that of Gasterosteus 

 among Thoracostraci and to that of such deep-bodied fishes as 

 Antigonia among the Acanthopterygii. The Opah differs from 

 typical Acanthopteri "in the absence of spines in the fins, and 

 the position of the ventral fins, together with the great number 

 of rays [14-17] in the latter, which is only met with in the lower 

 Teleosteans" (Boulenger). Gill suggests that the attachment 

 of the pelvis to the greatly enlarged hypocoracoids, as in the 

 Gasterosteidae, may be due to convergent evolution, and points 

 out that the Opah agrees with the Mackerel-like fishes in a 

 characteristic modification of the vertebrae and in "the deep 

 bifurcation of the roots of the caudal rays which clamp the 

 hypural and epural bones." The case is an instructive one as 

 illustrating the difficulty, without knowledge of the less special- 

 ized members of a group, of deciding whether resemblances to 

 some other group are genetic or convergent. 



Order Acanthopterygii 1 Cuvier 

 The Spiny-rayed Fishes. 



The structural characters enumerated under the superorder 

 Acanthopteroidei (page 497) are here seen in their most typical 

 condition. In addition, the opercle is always well developed, 

 the gill opening usually large and in front of the base of the pec- 

 toral fin, the scapula is typically perforated by the scapular 

 foramen, which may, however, appear between the scapula 

 and the coracoid. It is not certain whether the order is polyphy- 

 letic, or, as usually held, monophyletic and derived from Cre- 

 taceous Berycidae, which are generally conceded to be directly 

 ancestral to the Perciformes. The Berycidae retain such archaic 

 characters as an open swim-bladder, an orbitosphenoid, and more 

 than five soft rays in the ventrals. They are comparatively 

 numerous in the Upper Cretaceous and may conceivably have 

 given rise to the Stromateidae through Berycopsis, to the deep- 

 bodied Scombroids through the Pempheridae, and to the Scor- 



» ccKavQa, a thorn, rtrepvyiov, a fin, in allusion to the sharp spines in 

 the fins. 



