GREGORY: FISH SKULLS 441 



Adjustments to Different Types of Skin 



The varying characters of the basal layers of the skin have left their clear imprints on 

 the surface of the neurocranium, but the functional meanings of the patterns of pits, ridges, 

 etc., is far from clear and would well repay investigation. Contrast, for example, the dense, 

 closely pitted surface of the cranial shield of some of the large catfishes (Fig. 11) with the 

 cavernous, porous surface of the skull of the Sciaenidae (Fig. 125), in which the enlarged 

 mucous-secreting lateral line organs sink into deep pits. The varying texture of the surface 

 in the scorpsenoids has been described above (pp. 341, 343) and its connection with the 

 arrangements of the bony trabeculse noted. 



In Mola the skin has become very thick and forms a tough hide-like shield covering 

 the whole surface of the skull and body. This tissue has invaded and interpenetrated the 

 surface skull bones, burying them deep and profoundly modifying their texture (see p. 295). 



Spinescence 

 Whenever spines are developed, as on the surface bones of the skull and pectoral arch 

 (see p. 323) the phenomenon seems to involve the rapid proliferation of growing tips and 

 the laying down of relatively inelastic continuous layers so as to produce internal pressure 

 and crowding of layers. The location of these spinescent points frequently coincides with 

 the growth centers of the bones themselves and is evidently controlled by the hereditary 

 or genie pattern of the species. 



Development of a Cranial Buckler 



The development of a cranial shield has taken place quite independently in several 

 groups, such as certain families of siluroids (Fig. 80), the gurnards (Figs. 218-220) and 

 Dactylopterus (Fig. 223). In these cases some or many of the roofing bones become greatly 

 enlarged; they may grow backward over the neck {Dactylopterus) or a separate nuchal 

 shield may be developed (catfishes). In some of the gurnards the cranial shield becomes 

 heavy and seems to serve as ballast. For sluggish or partly sedentary fishes the protective 

 value of such a shield would seem to be high. 



Adjustments for Balance and Flotation 



Varying adjustments for balance and flotation have doubtless had considerable influ- 

 ence upon the skull in many and complex ways. In ChcBtodipterus faber, for example, the 

 great supraoccipital crest (Fig. 152) and interorbital bridge are much swollen and very dense, 

 so that they must measurably tend to depress the forepart of the fish and perhaps require 

 correctional movements of the pectoral fins or counterbalancing in some other parts, as in 

 the swollen epineural and interhaemal spines. Again, in the sheepshead Archosargus (Fig. 

 123), it would seem that the heavy dentition and jaws and the excessive amount of dense 

 bony tissue in the skull would overweight the head, if the body itself were not so huge. 

 The presence of a gas-filled air-bladder conditions an upward curve of the vertebral column 

 and this probably affects many details of the occiput. Apparently in high-backed fishes 

 the heightening of the back must tend to raise the center of gravity and thus increase the 

 instability. This is possibly compensated by lowering the center of gravity of the jaws, 

 throat and abdomen. Thus the position of the quadrate center (0, the angle of the gape, 

 the position of the eyes and the detailed manoeuvers in capturing the prey might all be 



