88 ILLINOIS BIOLOGICAL MONOGRAPHS [88 



of Polypterus, and going yet higher in the vertebrate series we find that two 

 pairs of connective tissue ossifications have been described attached to the 

 dorso-anterior margin of the cartilaginous supraoccipital in man, the interpar- 

 ietals and the preinterparietals (Ficalbi, 1890; Ranke, 1899). The latter are 

 inconstant, but the former may be the bones which correspond to the spina 

 occipitis of the teleosts and the supraoccipital plates of the Stegocephalans. 



McMurrich (1884b) states that in Amiurus the dorsal surface of the spina 

 occipitis is perforated with foramina for the passage of tubules connected v.dth 

 the 'mucous' canal system; an error since the canal system has no branches in 

 the posterior part of the cranium outside of the squamosal part of the squamoso- 

 pterotic. Further, he says that the 'ascending branch' of the first spinal 

 nerve (my hypoglossus) issues from the cranium through the foramen for the 

 ramus lateralis accessorius facialis. As I have not been able to find a dorsal 

 branch of this nerve in either the younger or the older specimens, I cannot 

 agree with him 



The exoccipitals. The floor and side walls of the foramen magnum are 

 formed by the paired exoccipital bones (Fig. 9). Nearly the whole margin of 

 each bone is smooth except for a few interdigitating spicules on the epiotic 

 and supraoccipital edges. It is separated anteriorly from the pterotic part 

 of the squamoso-pterotic and the posterior margin of the prootic bone by a strip 

 of cartilage which continues ventro-posteriorly between it and the antero- 

 dorsal margin of the basioccipital. On the posterior surface of the cranium it 

 is separated from the epiotic by the cartilage, but interdigitates with the ven- 

 tral margin of the supraoccipital as far as the wall of the foramen magnum 

 where cartilage is present between the bones. 



The anterior ventral surface of the bone is pierced by two foramina, a small 

 anterior one for the glossopharjoigeal and a larger one immediately posterior 

 for the vagus (Fig. 20). The ossification separating them is a very dehcate 

 osseous spicule and was originally cartilage. Just behind the vagus foramen 

 there is a sharp, upwardly curved prong, to which the transcapular bone is 

 attached. This latter bone has developed from the ossification of a ligament 

 between the shoulder girdle and the cranium (Fig. 37). Ventral to the articu- 

 lation of the transcapular with this bone, the surface of the exoccipital is rugose 

 for the attachment of the fibres of the shoulder girdle muscles. On the poste- 

 rior surface of the cranium the bone is concave medial to the base of prong, and 

 at the bottom of this concavity there is a minute foramen for the passage of 

 the hypoglossus nerve. A flange of the bone projects posteriorly behind this 

 foramen at right angles to the posterior surface of the cranium, forming part 

 of the lateral wall of the foramen magnum. This part of the bone develops 

 from the ossification of the membranous sheet, present in the younger stages, 

 posterior to the hypoglossus, and fuses with the ventral end of the occipital 

 arch. The posterior ventral margin of the bone is fused with the dorsal surface 

 of the thickened basioccipital. 



