416 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



the mouth itself. When we say that the mouth has become larger or smaller or has shifted 

 its position, what is really meant is that if we compare adult stages of successive geologic 

 ages we shall find that the parts around the mouth have expanded or contracted in such a 

 way as to cause the mouth itself to increase or decrease or to shift its direction or position. 

 Similarly, in the individual development changes in size or position of the mouth, as between 

 earlier and later stages, are doubtless due solely to the differential growth of cells surround- 

 ing the mouth. 



(1) Size. — If the mouth increases in size, it is because the jaw-bones, which lie on its 

 periphery, have been correspondingly lengthened. Here we note the existence of many 

 gradations, from the normal predaceous mouth of the salmon (Fig. 45) to the immense 

 cavernous opening of Gastrostomus (Fig. 94). In this case the jaws and hyomandibular 

 have become extremely long and slender hoops, by means of which the highly distensible 

 mouth, pharynx and stomach may be drawn gradually over the prey somewhat after the 

 fashion of the anaconda. In these forms, as well as in Cyclothone (Fig. 54), Chiasmodon 

 (Fig. 137) and others, the jaws and throat also act like a folding scoop-net. A very elabor- 

 ate apparatus of this sort (Fig. 176) combined with protrusile mouth, is found in the rare 

 Stylophorus (Starks, Tate Regan). As the mouth and pharynx increase in size it takes 

 relatively greater power to push them through the water at high speed. Hence the ad- 

 vantage of angulated orobranchial arches, which after the distension in the act of swallowing, 

 permit the folding-up of the apparatus, with consequent reduction in bulk and in resistence. 



In Lophius (Fig. 267), which is an animated fish-trap, the enormous jaws bristle with 

 pointed teeth. Very long jaws are operated at a mechanical disadvantage and are good 

 chieffy for a sudden rush or a quick snap. Thus their teeth are usually long and narrow. 

 Long teeth may be dagger-like and capable of inflicting severe gashes, as in Sphyrcena (Fig. 

 141) and Omosudis (Fig. 89), or very long and needle-like, in order to penetrate easily to 

 vital parts of the living prey, as in Chauliodus (Fig. 55). 



Small mouths, with accompanying reduction in the dimensions of the jaws and related 

 parts, seem always to have arisen by progressive reduction of normal jaws of a predaceous 

 type. Thus, for example, the small mouths of the Fundulus group (Fig. 96) appear to 

 have been derived from less minute mouths of the type seen in the Umbridse, which type 

 in turn is connected with the normal predaceous mouths of the typical Esocidae, Encho- 

 dontidse and Iniomi. In the actinopterygian fishes as a whole there are many gradations 

 (Fig. 283) from the primitive predaceous type downward to the small mouths of sparids, 

 hoplegnathids, pomacentrids, cichlids, labrids, scarids, chaetodonts, teuthids, acanthurids, 

 plectognaths, etc. 



(2) The position and direction of the mouth are likewise determined by the form and 

 arrangement of the jaw parts. The primitive mouth of the palseoniscoid Cheirolepis (Fig. 

 12) is almost terminal and directed slightly upward. From this developed, independently, 

 the peculiar mouths of the sturgeons (Fig. 19) and of Gonorhynchus (Fig. 65), which are 

 inferior and directed downward. A downwardly turned oval sucking-disc is developed in 

 some of the cyprinids, loaches and in certain catfishes (Fig. 80). The opposite specializa- 

 tion in which the mouth is superior and directed more or less directly upward is seen in 

 many groups, including the Microcyprini (Fig. 96), the extinct ichthyodectids (Figs. 35, 36), 

 the deep-sea Argyropelecus (Fig. 52), Chauliodus (Fig. 55), the ceratioids (Figs. 272, 273), 

 and in many bottom-living fishes such as Lophius (Fig. 267), the batrachoids (Fig. 263), 



