524 H. H. SWINNERTON. 



sends down a crest of bone (figs. 36 and 37) which lies 

 between and under the back processes of the paraspheuoid 

 (fig. 86, ps.) . Within the substance of the fore-part of this 

 crest is a cavity (figs. 36 and 37, ey. c.) which opens in front 

 and receives the hinder end of the external rectus muscle ; 

 this is the homologue of the anterior conical excavation of 

 the basioccipital of the pike (Huxley, 71, p. 133) and of many 

 other Teleosts. 



The exoccipital is separated from its fellow in the middle 

 line, above by the supra-occipital, and below by the basi- 

 occipital (PI. 29, fig. 24). The facet for articulation with 

 the first neural arch present in so many bony fishes is quite 

 unrepresented. The internal plate, which in the majority of 

 Teleosts (Klein, 84, p. 131) shuts out part or all of the 

 basioccipital from forming the floor of the cavum cranii, is 

 here not altogether absent, as Klein states (85, p. 133), but is 

 represented by a slight ingrowth extending anteriorly over 

 the basioccipital (PL 30, fig. 36, oc. e.). 



The ninth and tenth nerves no longer have a common exit, 

 but the process which commenced in Stage IV (fig. 12) has 

 given rise to two foramina widely separated from one another 

 (fig. 21, IX, X). In Amia, in which these are both present, 

 that for the ninth lies just behind the pro-otic and bears no 

 relation to the exoccipital, that for the tenth between this 

 and the opisthotic (Bridge, 77, p. 4). By coming to include 

 the vagus foramen it evidently trespasses somewhat on the 

 region of the opisthotic. In the part which immediately 

 surrounds the foramen magnum ossification has continued to 

 extend into surrounding tissue, so that the second occipital 

 nerve (fig. 19, oc. n.") now has a foramen also. In this 

 region in Amia both Bridge (77, p. 611) and Sagemehl 

 (84, p. 194) recognised the homologues of two, or possibly 

 three, vertebral arches; Gregenbaur (87) shows that the 

 elements they refer to are present in some, but not in all 

 Teleosts. If the two occipital nerve foramina in the 

 stickleback are indicative of two originally distinct arches, 

 it seems strange that whilst the first true vertebral arch is 



