66 ILLINOIS BIOLOGICAL MONOGRAPHS [66 



phylogenetic changes, have sunken to their present position and fused into a 

 solid osseous mass. Acipenser typifies the indifferent stage where the scales 

 have not yet formed bones. y 



The supraorbital lateral line canal is usually associated with the frontal bone 

 in the ganoids and teleosts, and there are many or few pores on the dorsal 

 surface of the bone connected with it. Vrolik (1873), in his general description 

 and in the conclusions of his work on the development of the frontal bones of 

 teleosts, states that the frontal bone is developed primarily to protect the canal. 

 This has since been refuted by those who have worked out the developmental 

 relations of the canal and the bone. The ossification around the canal is at 

 first entirely separate from that of the main part of the frontal (Klaatsch, 

 1895, Schleip, 1903, and others). In Amiurus, as remarked earlier in this 

 paper these have been noted as separate ossifications. 



In brief, the frontal bones of the teleosts are paired ossifications arising 

 from fibrous connective tissue. They may lie above a soHd cartilaginous roof 

 or they may form an integral part of the cranial roof. Anteriorly they usually 

 interdigitate vnih the supraethmoids and the ectethmoids, and are separated 

 from the nasal bones by a connective tissue bridge across which each supra- 

 orbital canal extends to enter the frontal. Posteriorly they usually interdigitate 

 with the parietals, but in the Siluroids the parietals are not present as discrete 

 ossifications, so they interdigitate with the supraoccipital. There is commonly 

 a fontanelle between the posterior ends of the bones, and in Amiurus and some 

 few of the Characinidae there is an anterior fontanelle as well. The frontals 

 overlap the orbitosphenoid and alisphenoid bones in the wall of the orbit, both 

 internally and externally in those forms where ossification has proceeded very 

 far. They also contain foramina and canals for the passage of the ophthalmic 

 branch of the facialis to the integumental sense organs on the dorsal surface 

 of the head and to the lateral line canal organs within the supraorbital canal. 



The infraorbitals. This series includes the lacrimal described above, and 

 another group of bones which extend from the posterior margin of the lacrimal 

 below the eye, so that the most postero-dorsal bone of the series, the postfrontal, 

 is attached to the frontal ridge, posterior to and above the eye (Fig. 15). The 

 whole series is made up of three suborbitals, two postorbitals, and the post- 

 frontal. These bones enclose the infraorbital or suborbital lateral line canal 

 and are developed primarily for its protection. None of the bones unites by 

 suture to its neighbors, but connexion is effected by ligamentous tissue and the 

 fascia enveloping the muscles of this region. 



The three suborbitals are the most slender and reed-like of the entire series. 

 They lie deeply embedded in the connective tissue anterior and posterior to the 

 ectethmoid process, two being anterior to the process and the third just behind 

 it. The first is the smallest of the trio and the second is next in size. Both of 

 these have practically the same diameter as the enclosed lateral line canal. 



