64 ILLINOIS BIOLOGICAL MONOGRAPHS [64 



the two sides of the head have fused around the epiphysial bar, and the carti- 

 lage within it has all but disappeared. None of this ossification connecting 

 the frontals of the two sides is perichondrial. In the adult, the cartilage has 

 entirely disappeared from the interior of the ossification (Fig. 7) surrounding the 

 original epiphysial bar. Anterior to this bar the frontal is thin and solid, while 

 behind, although just as solid, the bone is much thicker. The nerve foramina 

 in the bone are later developments and are caused by the growth of osseous 

 trabeculae around the nerve twigs and branches. The ossification around the 

 lateral line canal has now become an integral part of the bone. 



The older comparative anatomists homologise the frontals of the fishes with 

 those in the higher animals and based their conclusions upon topographical 

 rather than embryological relationships. As far back in the literature as I have 

 gone, these bones have always been known as the frontalia with, perhaps, an 

 added adjective to distinguish them from the anterior and posterior frontalia. 

 In practically every teleost and most of the ganoids they are the largest bones 

 on the dorsal surface of the cranium and are usually paired. Fontanelles like 

 those of Amiurus were recognized in the teleosts by Cuvier and are again 

 referred to by Stannius (1854). In Amia and most lower teleost families these 

 fontanelles are absent and the frontals are connected by suture along their 

 entire length. 



In comparison with the frontals of Amiurus those of Amia show limitations 

 of development ventrally and internally. They cover more of the dorsal sur- 

 face of the cranium than those of Amiurus, but they take no part in the wall 

 of the cavum cranii, being separated from it by the solid cartilaginous tegmen 

 cranii (Sagemehl, 1884). They do not interdigitate anteriorly with the supra- 

 ethmoid because of its limited development, but their relations to the nasal 

 bones are comparable to those of Amiurus. In the orbital wall, the frontal of 

 Amia is separated from the orbitosphenoid and alisphenoid ossifications by 

 cartilage. Cartilage does not extend between the frontal and ectethmoid 

 (prefrontal, Sagemehl) as in Amiurus. 



In the Characinidae there is a mixture of the condition found in Amia and 

 that of Amiurus. Citharinus (Sagemehl, 1885) approaches the Amiurus type 

 of frontal development externally, but is more like Amia internally, in that 

 more cartilage persists in the side walls and the roof of the cranium than in 

 Amiurus. The epiphysial bar is not enclosed by the frontals, although they 

 meet above it. Sarcodaces lacks the anterior fontanelle foimd in Citharinus 

 and the posterior fontanelle lies more between the parietals than the frontals. 

 Internally, there is less cartilage than in Citharinus, but the epiphysial bar 

 remains unossified. The other families of the lower teleosts — Mormyridae, 

 Osteoglossidae, Clupeidae, Gymnarchidae and others — have the frontals con- 

 nected by suture as far back as the parietals which are highly developed in 

 these forms. The internal relations of the frontals have not been described for 



