438 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



exoccipital from the epiotic and pterotic and prootic. These triradiate sutures seem to be 

 the result of peripheral growth from three ossific centers, which have thus moved away 

 from each other at nearly equal rates. 



Adjustments of the Neurocranium to Various Types of Jaw, Different Positions 



OF Mouth, etc. 



The neurocranium has solely a passive function in resisting stresses, but certain parts 

 of the branchiocranium, especially the jaws, teeth and suspensorium, which are compound 

 levers, assume an active function in modifying the neurocranium. For example, in the 

 sparids (Fig. 123) the front teeth become strong incisors, the tooth crowns on the side of the 

 premaxilla become obtusely conical to massive and hemispherical, and the same is true of 

 the teeth on the dentary; the fish is thus able to crush the shells of bivalves. We have 

 noted above (p. 249) the great enlargement of the maxilla and palatine to support these 

 unusually heavy shocks; hence it is not surprising that we find equally great massiveness 

 and strength in the construction of the cranial supports, namely the ethmo-vomer block, 

 the interorbital bridge, the cranial vault and the keel-bone. 



In large-mouthed, predaceous types the neurocranium must be variously strengthened 

 in accordance with the particular type of specialization. In the morays, for instance, the 

 neurocranium (Fig. 825) is long and narrow but very strongly built in order to support the 

 huge backwardly-developed hyomandibular and swollen branchial syringe, whose dorsal 

 muscles are fastened to it. The ethmo-vomerine block and interorbital bridge are likewise 

 stiffened in reaction to the stresses received through the enlarged vomerine teeth. In the 

 gulpers (Fig. 94) the neurocranium is remarkably wide and short because it has to be the 

 fulcrum for the very thick neck and back muscles and to carry the great living scoop-net 

 which is the mouth and pharynx. 



Again, the position of the mouth affects the neurocranium. In the sturgeon, for 

 example (Fig. 20), the interorbital bridge and ethmo-vomer block are drawn downward and 

 forward so that the lower surface of the ethmo-vomer block lies much below the level of 

 the floor of the cranial vault. Consequently the tactile ventral surface of the rostrum 

 can reach the muddy base in search of food immediately in front of the small suctorial 

 mouth. This, however, does not explain why the neurocranium of the sturgeon has become 

 secondarily cartilaginous. A dorsal displacement of the mouth naturally affects profoundly 

 the ethmo-vomer block and the anterior part of the keel-bone but is not often without 

 marked indirect effects upon the cranial vault. In Pelor japonicus, for example (Fig. 206), a 

 strongly modified scorpaenoid, the wide mouth is pushed upward and backward to such an 

 extent that the ethmo-vomerine block is greatly shortened and widened; as the eyes also 

 are turned upward and pushed backward, the cranial vault is considerably widened and 

 shortened. 



The opposite specialization is seen in the tube-fishes (Fig. 294C), in which the mouth, 

 although equally upturned, has been drawn very far forward, greatly elongating the ethmo- 

 vomer block. Here also (Fig. 230C) the interorbital bridge has shared in the elongation 

 but to a much less degree. In the balistids also (Figs. 160-164) the ethmo-vomer block 

 shares in the forward growth of the suspensorium and affords a firm support for the small 

 but powerful nipping mouth. 



The branchial apparatus as a whole is but loosely connected with the neurocranium 



