62 ILLINOIS BIOLOGICAL MONOGRAPHS [62 



ethmoid, and ectethmoid — and in some of the fishes with the vomer. Allis' 

 (1898) criterion for the homology of the lacrimal is the inclusion within it of 

 the anterior end of the suborbital canal. In comparing his work on Amia with 

 that of McMurrich on Amiurus, he concluded that the antorbital of Amia was 

 the homologue of the adnasal (lacrimal, Author) of Amiurus. Since however, 

 this bone of Amiurus contains the anterior end of the suborbital canal it is the 

 lacrimal and the antorbital of Amia is represented by the long antero-lateral 

 process, which has fused with the lacrimal element as it has in some other 

 teleosts. 



The frontals. These are the most extensive and conspicuous bones on the 

 dorsal surface of the cranium, forming most of the roof and part of the side 

 walls of the cavum cranii (Fig. 10). The only point of suture between the two 

 is in the region of the original epiphysial bar. Anterior and posterior to this 

 suture they are separated from each other by two longitudinal fontanelles, the 

 remnants of the more extensive ones of the younger stages. Each has a raised 

 margin on the sides of these fontanelles, thus bounding a fossa which continues 

 anteriorly as far as the supraethmoid and posteriorly on to the dorsal surface of 

 the supraoccipital. This fossa is filled with connective tissue and nerve fibres, 

 and a tough membrane is stretched the entire length of each of the fontanelles. 

 Their restriction is caused by median growth of the frontals, a process noted 

 in its incipience in the 32 mm. stage. Earlier, however, the posterior fonta- 

 nelle extended between the anterior edges of the occipital arch, but the ossifica- 

 tion in this region has growTi forward and closed the extreme posterior end of 

 the fontanelle. 



Anteriorly, the frontals interdigitate with the median supraethmoid, which 

 forms the anterior margin of the anterior fontanelle. This interdigitation lies 

 above the orbito-nasal foramen and is continuous laterally with that between 

 the ectethmoids and the frontals (Fig. 10). An oblique frontal ridge, along 

 which the adductor mandibularis muscle has its origin, extends posteriorly across 

 the dorsal surface of the frontal from the lateral posterior margin of the ecteth- 

 moid. The frontal ridge does not extend to the posterior end of the bone, but 

 unites with the ridge which forms the margin of the median fossa. Behind 

 the union of the two ridges a wing of bone extends laterally to interdigitate 

 with the sphenotic. Anterior to the ridge the surface of the bone is sculptured 

 in longitudinal lines, which run posteriorly toward the middle line of the 

 cranium, parallel to the dorsal margin of the ridge and extending along its 

 anterior face. Posterior to the frontal ridge the bone is comparatively smooth. 



At the anterior end of the frontal ridge the suborbital lateral line canal enters 

 the frontal bone from the postfrontal (Fig. 11). The two bones are not in con- 

 tact with each other, so that the canal crosses the dorsal surface of the adductor 

 mandibularis muscle before entering the frontal. A dermal tubule is given 

 off from this part of the canal as it passes from the postfrontal to the frontal. 



