GREGORY: FISH SKULLS 189 



cular. The latter Is somewhat inclined forward. It is much larger than the opercular 

 which, however, has a large glenoid process for the pedicle of the hyomandibular. 



This very peculiar skull exhibits no obvious marks of near relationship with that of 

 Eigenmannia, but somewhat intermediate conditions are found in Gymnotus carapo (Ellis, 

 PI. XVI) and Eigenmannia virescens (PI. XVHI). In Rhamphichthys rostratus (PI. XVII) 

 the snout is very long, the suspensorium much produced and the mouth very small, the 

 whole recalling somewhat the form of the head and snout in certain mormyrids. The 

 elongate preopercular is almost horizontal. 



The subhorizontal position of the preopercular in such short-headed forms as Eigen- 

 mannia, together with the fact that the hyomandibular process of the opercular points 

 upward rather than forward, suggest that the primitive gymnotid had the middle part of 

 the skull at least fairly elongated, and that the shortening of the skull in Eigenmannia and 

 many other genera is quite secondary. This view is also consistent with the graphic 

 diagram of generic relationships given by Ellis (PI. XV). 



The amazingly wide range in skull structure in this group and the indubitable evidence 

 of relatively close relationship of all its genera and species indicate that it is now undergoing 

 a rapid evolutionary expansion, along with its parent group the characins. 



With regard to the otoliths, Frost (1925^, p. 556) notes that "The otoliths of the 

 Gymnotiformes are not far removed from those of the typical Characiformes, from which 

 type they have apparently been derived." 



EVENTOGNATHI (CaRPS, SuCKERS, ETC.) 



The typical carp skull (Fig. 74) is more specialized than that of the primitive characin 

 in the edentulous and highly protrusile character of the upper jaw. The mechanism of the 

 carp's mouth was carefully described and figured in 1886 by Vitus Graber, whose work is 

 cited by Thilo (1920). The latter gives a very clear brief account of the origin and "de- 

 generation" of this mechanism. Sagemehl (1891, p. 583) described the circumoral struc- 

 tures of the carps, suckers and loaches chiefly from the morphological viewpoint, while 

 Starks (1926a) gives many important descriptive details of the ethmoid region, including 

 the attachments of the "submaxillaries" in these families. L. F. Edwards (1926) has 

 recently published an excellent description of the protractile apparatus of the mouth of 

 the catostomids. The following brief description is based primarily on the above men- 

 tioned papers but with constant reference to preserved specimens and skulls of various 

 cyprinids and catostomids. 



As noted by Sagemehl (1891, p. 583), the protrusile condition of the jaws has doutbless 

 been acquired independently of that of the percoid fishes, from which it differs in important 

 details. 



In the typical "suckers" (Catostomus) the toothless premaxillae and maxillae are hidden 

 in thick circular lips. In the carp (Fig. 74) the lips are not so thick but the edentulous 

 jaws are equally protrusile. The premaxillae have ascending processes to which are 

 attached by ligament a slender median tracker, the "rostral bone" of Starks (1926a), or 

 "preethmoid bone" of Edwards (1926). This bony tracker in turn is attached posteriorly 

 by a long flexible ligament to the anterior end of the mesethmoid just above the notch 

 (Fig. 75^) between the anterior forks of the vomers (Starks, 1926a, p. 173). When the 

 jaws are retracted and closed, the bony tracker and its posterior ligament are folded up into 



