32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



not have a protrusile upper jaw. On balance, however, there are 

 probably more fish species living today that have a protrusile upper 

 jaw than there are species that do not. 



The fact, noted above, that many fishes have lost the protrusile pre- 

 maxillaries often makes it difficult to state definitely that a fish with a 

 fixed jaw never had a protrusile jaw. 



In the percoid type of jaw protrusion the mesethmoid and vomer 

 have a median ridge (fig. 8B) which forms a rail along which the 

 premaxillary heads (pedicels) slide. This prohibits lateral dislocation. 

 The premaxillaries are held pressed to the rail by a series of liga- 

 ments. At the front the two premaxillaries are bound to one another, 

 and anterolaterally are held in place by the palatine-premaxillary liga- 

 ment (fig. 8C) ; posteriorly they are firmly tied to the maxillary and 

 the lower jaw. 



When a fish with protrusile premaxillaries of the percoid type 

 opens its mouth, the upper jaw automatically slides forward and 

 downward along the ethmovomerine ridge. The anterior portions of 

 the premaxillaries are wedged forward by the outward rotation of 

 the anterior maxillary heads ; this rotation is associated in turn with 

 movement of the forward end of the palatine (van Dobben, 1935). 

 The posterior ends of the premaxillaries are forced forward and 

 downward by the lowering of the mandible. 



Thus protrusion of the premaxillaries is accomplished by a rather 

 complex mechanism. Nevertheless, all the basic elements necessary 

 for the percoid type of premaxillary protrusion would seem to be 

 present at least as far back as the basal scopeliform fishes, e.g., 

 Aulopus. Furthermore, even in such clupeiform fishes as Megalops 

 and Chipea (Kirkhoff, 1958) the whole system of ethmoid-palatine- 

 maxillary-premaxillary articulations and ligamentous connections is 

 present and the premaxillaries are to some extent automatically ro- 

 tated, if not protruded, when the mouth is opened. In actual practice 

 the only criterion the author has been able to find for whether the 

 premaxillaries can or cannot be protruded is the presence or absence 

 of an infolding of the skin across the front of the snout behind the 

 upper jaw : where a broad or narrow f renum is present between the 

 cranium and the upper lip, it is assumed that the premaxillaries are 

 nonprotrusile. 



The teleostean orders (following Berg's 1940 classification) in 

 which a protrusile upper jaw is present are the following: Clupei- 

 formes (Gonorhynchns (?) and Phractolaemus only), Bathyclupei- 

 formes, Ateleopi formes, Cyprini formes (Cyprinoidei), Gadi formes, 



