326 HOLOSTEI 



wards to the nasal sacs (Figs. 353, 508); in it run the olfactory 

 tracts (Gadidae) or nerves (Macruridae). It is probable that similar 

 modifications have taken place independently in several families ; 

 but what is not likely is that fish which have once reached one 

 extreme form (with sessile bulb) should be able to give rise to genera 

 which exhibit the other extreme. In the classification of such a 

 modern group as the Teleostei, where every scrap of evidence is of 

 value, the relation of the brain to the septum is therefore of great 

 importance. The median septum of the Lepidosteoidei, chiefly in 

 front of the orbits, is probably due to the elongation of the snout 

 (ethmoid septum), and not strictly homologous with that of the 

 Teleostei, 



Another new formation in the Holostean skull is the so-called 

 eye-muscle canal, or myodome (Figs. 302-5, 358). In all fish the four 

 recti muscles are attached close together to the inner wall of the orbit 

 near the basis cranii in the basisphenoid region. Now in Amia and 

 many Teleostei these muscles pass farther inwards and backwards 

 into a space hollowed out in the floor of the cranium. This canal is 

 arched above by transverse wings of the prootics, and its roof is 

 completed by membrane in front and at the sides, so that it does not 

 really open into the cavum cranii. Its floor is formed either by 

 the prootics (Amia) or, when these fail to meet in the middle- 

 below, by the parasphenoid (many Teleostei). Moreover, in the 

 latter case when the canal extends far back it may be closed 

 above by the alisphenoid, prootic, and basioccipital, below by the 

 parasphenoid ; but in the dried skull the canal may open behind 

 between the basioccipital and the parasphenoid (Clupea, Elops, 

 Salmo t etc.). 



Lastly, the occipital region is of interest. In no fish below the 

 Teleostei is a median supraoccipital bone known to occur (Figs. 238, 

 305, 329). Superficial dermal occipital plates are often found, and 

 may possibly have given rise to the supraoccipital, but there is no 

 evidence that it is derived from such plates. Rather does the 

 supraoccipital seem to have been developed from the neural spines of 

 one or more of those vertebrae that have last been incorporated in 

 the occipital region of the skull ([497] and p. 11). A difficulty 

 encountered by this view is the fact that already in Lepidosteus 

 (Shreiner [390a]) some six or seven sclerotomes have been included 

 in the skull without the appearance of a bony supraoccipital, and 

 there is no evidence that the Teleostean skull holds more segments 

 (Fig. 322). Ontogeny supports the view that it is an endo- 

 chondral bone. The limit between the skull and the vertebral 

 column in the lower Holostei is somewhat indefinite : in Lepidosteus 

 and many Teleosts one, and in Amia two occipital neural arches 

 rsmain separate while their corresponding centra fuse with the 

 occipital region. It is, of course, quite possible that the supra- 



