90 ILLINOIS BIOLOGICAL MONOGRAPHS [90 



sists between them. In this same group of fishes there is usually a Ugament 

 extending from the basioccipital to the shoulder girdle (Sagemehl, 1885), 

 which he says is the homologue of the transcapular process of Amiurus. 



The synonomy of this bone has been given by Owen (1848), Vrolik (1873), 

 and Starks (1901). To their hst of synonyms may be added the name given 

 to this bone by Gaupp (1906), pleuroccipitale. Of all the names, exoccipital, 

 suggested by Owen, is the briefest, and most comprehensive of the relations of 

 the bone; pleuroccipitale has no excuse. 



The basioccipital. This is the most posterior bone on the ventral surface 

 of the cranium and is fused on its ventral face to the anterior centrum of the 

 compound vertebra of the spinal column. The bulk of the bone lies in this 

 immediate region and thins out anteriorly on the ventral and lateral surfaces 

 of the cranium (Fig. 6). Its ventro-anterior margin is overlapped by spicules 

 of the parasphenoid and its dorsal anterior surface by the posterior margin of 

 the suprasphenoid and the prootic (Fig. 7). The dorso-lateral margin of the 

 anterior end of the bone interdigitates externally with the extreme posterior 

 ventral margin of the prootic (Fig. 20). Behind this interdigitation the 

 basioccipital is separated from the antero-ventral margin of the exoccipital by 

 cartilage until the two exoccipitals fuse posteriorly. Below this fusion the poste- 

 rior face of the basioccipital is circular and deeply concave (Fig. 9). The edges 

 of the concave anterior face of the centrum of the first vertebra is fused with 

 the periphery of this face of the basioccipital and in the space enclosed between 

 the faces is filled with gelatinous notochordal tissue. A small space in the ven- 

 tral surface of the basioccipital just anterior to its fusion with this centrum 

 remains unossified and is filled with cartilage, the remnant of the hypochordal 

 cartilage of the 32 mm. stage (Fig. 38). 



The internal surface of the bone is hollowed out for the reception of the 

 sacculi. It forms the median crest between the recessus sacculormn and part 

 of their floor and side walls. The dorsal surface of the crest is concave, form- 

 ing the floor and walls of the ca\aim sinus impar. In the side wall of each 

 recessus the internal basioccipital lamella meets the descending lamella of the 

 exoccipital of that side. In the 32 mm. stage this had already completely 

 ossified and so in the adult there is no cartilage left in the walls of the recessus 

 except between the margins of the bones. Considerable cartilage yet remains 

 in the basal plate between the posterior end of the prootic and the basioccipital 

 (Fig. 7). There is no trace of the intercranial notochord so prominent in the 

 younger stages. 



The transscapular bone has part of its attachment to the !ateroventral 

 surface of the basioccipital (Fig. 9). 



The basioccipital of the teleosts is very constant in its morphological rela- 

 tions. As in Amiurus, its posterior face is concave where attached to the 

 centrum of the first vertebra and the gelatinous mass is nearly always present 

 between them In the Ostariophysi the sacculi penetrate more deeply into 



