SkuU of Gonoiliynclms Greyi. 371 



front; but, on tlie otlier hand, it possesses two surmaxillae 

 and an ossified first pliarynji^obranciiial in addition to tlic 

 spicular. Alepocephalus resembles Gonorhi/nc/nis in possess- 

 ing: J^'i epibranchial ()r<j:;an, borne by tlie fourtli and fifth 

 arches, and in possessing a cartihige which may be identified 

 as the fifth epibrancliial ; but the list of resemblances is soon 

 exhausted. 



On the other hand, the Salmonidse, though ofi^'ering no 

 close resemblances to the GonorhynchidiB, consist of a variety 

 of forms but little specialized and highly plastic. For the 

 purposes of comparison the genus Salmo is less suitable than 

 such a form as Curegonus, for the Salmons have an excess of 

 cartilage, presumably of secondar}^ origin, in the cranium, 

 and no membranous interorbital septum such as Coregonus 

 has. It may be pointed out that within the family Salmouidie 

 there are forms, such as Coregonus oxyrhynchus, with promi- 

 nent snout and reduced mouth with no teeth. 



Although a study of the cranial osteology of the Gono- 

 rhynchidfe and Salmonidse cannot bring forward direct 

 evidence of affinity between these families, the hypothesis of 

 the descent of the Gonorhynchidae from the Salmonoid st )ck 

 is open to little objection of any serious import. The 

 Salmonidse have an ossified first basibranchial, whereas this 

 element of the copular skeleton fails to ossify in Guno- 

 rhynchus; but, as already shown, this basibranchial behaves 

 in its ossification in a most capricious manner in admittedly 

 closely allied genera. The Salmonidte have no epibranchial 

 organ ; but this organ, as I have indicated in a former paper 

 (Proc. Zool. Soc, 1904', ii. p. 81), has certainly been evolved 

 independently in a number of different groups of fishes, and 

 in these exhibits such differences in structure and position 

 with regard to the parts of the branchial skeleton that one 

 may reasonably allow that the Gonorhyuchid?e have developed 

 their epibranchial organ since their separation from the 

 ancestral stock of the Malacopterygii. 



EXPLANATION OF PLATE XVI. 



Fig. \. G onorhynchus Greyi; hyopalatine arch of the left side, with pie- 



opercular bone and mandible, mesial aspect. 

 lig. 2. Cranium, seen from left side. 

 Fig. 3. Back view of cranium. 

 Fig. 4. Dorsal view of cranium. 

 Fig. o. Complete skull, right side. 



species of Coregonus as I have been able to examine, and also in Salmo 

 Mappii, it is closed posteriorly in Salmo hucho. 



