GREGORY: FISH SKULLS 437 



each thrust-stream would be divided and distributed, one main division passing upward, 

 inward and slightly forward toward the occipital crest, a second upward and forward to 

 divide again into two branches, one leading to the epiotic process, the other to the posterior 

 process of the pterotic. The forward thrusts on the basioccipital spread out radially on the 

 basioccipital in a wide V but from the tip of the V a median tract with parallel sides runs 

 forward to join the parasphenoid; apparently this median tract serves to resist tension 

 rather than compression, and indeed such tension could readily be excited (through the 

 intervention of the parasphenoid) by upward pressure on the under surface of the vomerine 

 tooth patch. 



The architecture of the roof of the cranial vault is no less challenging than that of the base. 

 The interorbital bridge, including part of the frontals and alisphenoids, forms a beautiful 

 arch, the thrusts of which are transmitted through spreading postorbital pillars, which rest 

 on the prootics, the sphenotics and the pterotics. Between these diverging limbs at the 

 right and left sides is a large median fissure, bounded below by a median keystone of an 

 inverted arch formed by the basisphenoid. The median fissure aff'ords a secure and im- 

 mobile seat for the front part of the brain, including the olfactory stalks. Lateral to the 

 base of the arch lies the trigemino-facialis chamber for the great ganglia of the trigeminus 

 and facialis nerves, which are protected from being crushed by means of small but relatively 

 strong pillars formed by the prootic bone. 



The support for the hyomandibular, which has to withstand much of the forces that 

 tend to wrench, push or pull the inner and outer jaws, is very appropriately located (Fig. 

 117) between the powerful postocular arch just described and the occipital buttress afforded 

 by the lateral wing of the pterotic. The socket for the hyomandibular is always tripartite, 

 with the sphenotic and prootic parts in front, the pterotic part in the rear. The prootic 

 facet is supported by a pedicle that runs downward and forward between the facialis and 

 the trigeminal ganglia, lateral to the trigemino-facialis chamber. The sphenotic facet is 

 usually supported by the heavy postorbital process of the sphenotic, the pterotic facet is 

 horizontally extended backward above the top of the opercular. This facet is supported 

 by a very strong buttress on the exoccipital that runs obliquely downward and backward 

 toward the exoccipital condyle, and sometimes by another smaller one that runs downward 

 and forward. 



The pterotic facet is often braced above by longitudinal ridges on the pterotic and parie- 

 tal and in the tarpon by a stout transverse ridge running from the pterotic to the parietal. 

 In the upper half of the occiput above the exoccipitals there are often deep fossae bordered by 

 ridges for the insertion of the trapezius muscles of the shoulder-girdle. In the tarpon (Fig. 

 32) and some other isospondyls these fossae are very deep. The supraoccipital crest sup- 

 ports the anterior extensions of the muscles of the dorsal fins. In many scombroids (Figs. 

 183, 192) this crest extends forward to the snout and is supported by a great horizontal 

 plate formed by the frontals. 



The inner surface of the braincase (Figs. 32, 119) consists broadly of: (1) depressions 

 for the various parts of the inner ear and of the brain; (2) elevated ridges and partitions 

 between these parts. Many of the above-named ridges and buttresses on the outer side 

 seem quite well adapted to resist tension as well as compression. 



The entire cranial vault is divided by a system of triradiate sutures (Fig. 299). One 

 triradiate suture separates the alisphenoid, sphenotic and prootic, another separates the 



