526 H. H. SWINNERTON. 



fontatiolle is very large (Sagemehl, 85, p. 41 ; 91, p. 512 ; 

 Pollard, 95, figures), and the posterior portion of the cranial 

 roof is not extensive. Hence the whole bears a close resem- 

 blance to the cranial roof of the yonng salmon (fig. 61) or 

 stickleback (fig. 3). The approximation of the parietals is 

 therefore a necessity for actual roofing purposes. In the 

 non-ostariophysous Teleosteans, however, the posterior cranial 

 fontanelle, judged by the development of the salmon, 

 stickleback, pike, Engraulis, and by the adult Alepocephalus, 

 always becomes divided by a more or less massive median 

 bridge of cartilage, and. either greatly reduced or completely 

 suppressed by the growth of this and. of the posterior cranial 

 roof (cp. fig. 4). Thus the supra-occipital ossification has 

 free scope to advance forward or laterally to any extent, and 

 consequently the necessity for the parietals as roofing 

 elements of the cranial cavity diminishes. For the same 

 reason these bones, as compared with the frontals, are 

 generally large in one group and small in the other. These 

 features in themselves are almost important enough to 

 separate the two groups, and while they lend great support 

 to the validity of the group Ostariophysi, they also support 

 Sagemehl's suggestion that these fish might have been 

 derived from a type even lower than Amia (85, 91). 



Concerning the origin of the supra-occipital, Sagemehl 

 (91, p. 523) sought to show that it was the dorsal ossification 

 belonging to the morphological first vertebral arch. Its 

 position does not, however, support this view, for both in 

 salmon and stickleback it arises, not between the upper ends 

 of the exoccipital, but further forward, between the epiotics. 

 In some cases (e. g. Pleuronectidas) it may be separated from 

 the foramen magnum not merely by the exoccipital but also 

 by the epiotics (Klein, 84, p. 131). Whatever its origin, as 

 Sagemehl pointed out (91, p. 521), it is the youngest bone in 

 the Teleostean skull, and has arisen wholly within this class. 



In the presence of a vertical ridge, and in the relation of 

 this to the exits of the fifth and seventh nerves, the sphenotic 

 region closely resembles that of Alepoc(>})halus. 



