182 ME. F. .1. COLE ON THE STRUCTUEE AND MOEPHOLOGY OF 



(1) Supra-orhital canal. — In Amia there are three on the nasal and four in the frontal, 

 of which the first nasal and the last frontal are those perhaps missing in Gadus. In 

 hoth types the sense organs are fewer than in any of the other canals. The second 

 dermal tubule of Amia is missing in Gadus, the fourth undoubtedly corresponds to 

 the 3rd of Gadus, whilst the 6th, 7th, and Sth of Amia are again missing. Indeed, 

 the Gadus supra-orbital is, for perhaps some functional reason, exceptionally abbreviated, 



(2) Infra-orhital canal. — I must first pi'otest against Allis's definition of this canal, 

 which is very nnphilosophical. He considers i-ightly the innervation of the lateral canals 

 to be the best criterion of their bovmdaries, and applies this method to all hut the infra- 

 orbital canal, yet ofi'ering no justification for the exception. Behind the operculo- 

 mandibular anastomosis the main canal is innervated by the lateralis lateral line nerve 

 (including the lateral line branch of the IXth, which belongs to the lateralis), and this 

 should determine the boundary of the body canal : in fact the operculo-mandibular 

 anastomosis with the main canal is situated between the infra-orbital and lateral canals. 

 Ewart (1892, 68) also criticises Allis's definition of this canal, and considers, Avith the 

 writer, that its posterior segments belong to the lateral canal, whilst Miss Piatt (1896, 

 158) agrees Avith Ewart and correctly limits the infra-orbital canal to the buccal nerve. 

 I hence consider Allis's sense organs 17-21 infra-orbital to be situated on the lateral or 

 body canal. 



The ethmoid and ant-orbital or j^re-orbital ossicles of ^m«« are unrepresented in Gadus. 

 Both are probably lateral line ossicles and do not belong to the skull sensu slricto — 

 the ant-orbital being doubtless a modified sub-orbital plate. This diff'erence makes 

 it difficult to homologise the sense organs of the two forms, but assuming that the 

 first post-orbital of Amia represents two fused ossicles, as seems probable, then sense 

 organs 9 . 10 . 11 . 12 . 13 Amia = 6 . 7 . 8 . 9 . 10 Gadus. Sense organ 14 of Amia in the 

 sjihenotic is absent in Gadus, but the course of the otic nerve in the latter form is 

 marvellously suggestive either of its having once existed there, or, what is perhaps more 

 probable, that it has shifted backwards and now forms the 11th sense organ on the j)terotic. 

 The otic branch of Amia innervates two sense organs, both on the pterotic, of which the 

 16th seems to be the one missing in Gadus, since the 16th dermal tubule is absent. 

 Dermal tubules 13 and 14 Amia are also wanting in Gadus virens, but 13 is present as 

 No. 9 in G. morrhua. The absence of 13 in G. virens is somewhat remarkable, but I 

 have not seen it in any of the specimens I have examined. The outer buccal nerve is 

 represented in Amia by the branches to sense organs 11 . 12 and 13 infra-orbital, with 

 perhaps the branch to sense organ 14. 



(3) Ryomandihular canal. — The dentary of Amia bears 7 sense organs as against 6 

 in Gadus, but it would be impossible to say which was absent in the latter genus. The 

 canal then enters in Amia what AUis calls in his first paper the " angular," but is some- 

 what doubtful about the point. The bone undoubtedly corresponds to the Teleostean 

 articular, which homology Allis has recognised in his last paper. Bridge's small ossicle 

 and Allis's ossicle a (1897, 6, figs. 1 & 2) are doubtless comparable to the Teleostean 

 angular. There are three sense organs in the Amian articular as against one in Gadus, 

 and six in the preoperculum as against five. In the latter case there can be little doubt 



