THE CEANIAL NEEVES AND LATERAL SENSE ORGANS OE FISHES. 123 



long, laterally compressed, oval median fontanelle, which measured 12 mm. long in the 

 present sjDCcimen. The posterior extremity of this fontanelle represents the nearest 

 approach which the supra-orbital canal makes to the mid-dorsal line, the two supra- 

 orbital canals being only separated at this region by a cylindrical bony basal projection 

 or septum 3 mm. wide. It is here that the two supra-orbital canals communicate Ijy a 

 well-marked transverse commissure, very obvious in sections of young Gadids, but apt 

 to be overlooked in dissections of adults. In front of this region the canal at once 

 liegins to course obliquely outwards, and continues to do so as far as its anterior 

 extremity. The bony septum above I'eferred to becomes wide at the anterior extremity 

 of the fontanelle, where it is nearly 5 mm. wide. The median closed portion of the 

 tube is a little over 10 mm. long, in front of which the next 14 mm. of the canal 

 are imperfect externally, becoming more and more so anteriorly. The course of the 

 supra-orbital canal from the posterior extremity of the median fontanelle up to tlie 

 anterior edge of the frontal has been slightly obliquely outwards and forwards. 



The fourth mipaired dermal tubule, which is the longest of those opening into the 

 supra-orbital canal, enters it at the short transverse commissure. The surface pore in 

 adults is, in all the specimens that I have examined, slightly to the left of the median 

 dorsal line, whilst the tubule itself opens precisely at the centre of the commissure, 

 although I have seen it also somewhat to the left. In sections of young Gadids, however, 

 both the surface j)ore and the internal opening are exactly median — another instance of 

 the slight disturbance of the symmetry of the lateral line organs which lakes place in 

 the adult. It may be mentioned here that this is the only unpaired dermal tubule in 

 the Cod. 



(2) Iiifra-orhital Canal. 



This will be described under the headings of the various bones which support it. 



First Suh-orhital or Lachrymal Bone *. — The infra-orbital canal arises blindly a little 

 in front of the lachrymal near the mid-dorsal line at the extremity of the snout; The 

 blind extremity exists from the very beginning, and is obvious both in sections of young 

 forms and in dissections of the adult. The anterior extremity of the lachrymal is grooved 

 to receive this part of the infra-orbital canal. The lachrymal itself is roughly triangular, 

 with the apex directed forwards. Its length is about 60 mm., and breadth at the base 

 over 35 mm. The base may be divided into two concave portions, separated by a median 



* The synonymy of the bones of the Teleostean skull is, it is hardly necessary for me to point out, a somewhat 

 obscure subject. I have endeavoiu-ed as fur as possible to steer a middle course, and therefore hope that no objection 

 can be taken to any of my terms. It is, perhaps, necessary to exjilain that Allis (6) and other authors consider the 

 first sub-orbital and lachrymal to he two separate bones. No reasonable objection, however, can be brought against 

 the view advocated by McMurrich (136), that the lachrymal is a modified sub-orbital plate, and I therefore describe 

 it as the first sub-orbital. "We may note in this connection that the Teleostean lachrj-mal has been erroneously 

 termed by some authors the pre-frontal. The correct homologue in the Teleosts of the Ganoid pre-froutal is, 

 however, the lateral ethmoid (=the parethmoid). For the rest, sphenotic and pterotic are undoubtedly synonyms 

 of post-frontal and squamosal respectively (but see p. 132). The confusion in the nomenclature of the bones of the 

 fish's skull has arisen partly by the misuse of some terms and partly by the unnecessary introduction of others. 



SECOND SERIES. — ZOOLOGY, VOL. VII. 18 



