MORPHOLOGY OF TELEOSTEAN HEAD SKELETON. 525 



preformed in cartilage, these are not. Nor does the develop- 

 ment and structure of the basioccipital or its enclosed 

 notochord show any signs that centra have been absorbed. 

 It may be, of course, that this is another case in which 

 during ontogeny features have been suppressed which were 

 present in the ancestors of this fish. 



The supra-occipital is an unusually large bone (fig. 24, oc. s.). 

 Anteriorly it seems to partially separate the frontals ; in 

 reality it extends under them (fig. 20) almost to the level of 

 the outstanding post-orbital processes, and now embraces the 

 larger part of the epiphysial cartilage itself also. It is quite 

 absent in Araia. In the salmon, pike, Alepocephalus, 

 Characinidse, and Cyprinidse it falls far short of that of the 

 stickleback in forward and lateral extension. But whilst in 

 Characinidas and Cyprinid^, the majority of which have more 

 completely osseous chondrocrania than the stickleback, the 

 parietals are never separated by the supra-occipital ; in other 

 physostomes — excluding the siluroids, in which the parietals 

 are either absent or are fused with the supra-occipital — there 

 is every gradation from the primitive Amioid condition, with 

 its parietals side by side, to the condition in which they are 

 widely separated. The former is seen, for example, in 

 Osteoglossum, Araparima, and many Clupeoids ; the tran- 

 sitional forms are seen in some Salmonoids, and the latter 

 condition is seen in the rest. In the case of Stenodus, a 

 Salmonoid, the separation or non-separation seems to be 

 merely amatter of age, for in young individuals the parietalsare 

 apart, in old ones they are in contact (Boulenger, 95, p. 299). 



Allowing for the existence of the peculiar fenestras between 

 the frontals and parietals in the Ostariopbysi, the constancy 

 of the approximation of the parietals in them, on the one 

 hand ; and the inconstancy of the same feature, or the general 

 separation of these bones in the non-ostariophysous fishes, on 

 the other ; ai'e but superficial expressions of a fundamental 

 difference between the chondrocranial roofs of these two 

 great groups. In the former, the supra-occipital can never 

 extend far forwards, because in the adult the post-cranial 



VOL. 45, PART 4. NEW SERIES. N N 



