126 ME. F. J. COLE ON THE STEUCTUEE AND MOEPHOLOGY OF 



tube is formed on the pterotic by a latero-external process being sent down. Posteriorly 

 this process only forms an imperfect tiibe, but anteriorly it usually almost completely 

 encloses the canal, yet does not fuse ventrally with the floor of the ossicle. The tube 

 is thus imperfect for the whole of its length, but less so anteriorly than posteriorly. The 

 last or eleventh dermal tubule of the infra-orbital canal opens into it between the pterotic 

 and the adjoining supra-temporal, but the greater part of the aperture is on the pterotic. 



(3) Hi/omandihula r Gmial. 



The hyomandibular canal arises on the dentary at its anterior extremity near the 

 symphysis, its length, whilst on this bone, being about 80 mm. The direction of the canal 

 depends, naturally, on the position of the lower jaw. When the jaw is closed the canal 

 has a stioug upward incline. Its relations with the articular portion of the canal are, of 

 course, fixed, and do not vary with the gape of the jaw. The hyomandibular canal fits 

 into a deep groove on the ventral surface of the dentary which is continued almost to the 

 anterior symphysis. This groove is widely open ventrally, and gradually tapers towards 

 its anterior extremity. The first five dermal tubules open into the hyomandibular canal 

 whilst on the dentary, the sixth opening at the junction of the dentary and articular, 

 but rather on to the dentary. 



Leaving the dentary the hyomandibular canal passes on to the articular, and then 

 begins the upward curve which carries the canal on to the pre-operculum. Strong dorsal 

 and ventral ridges form a stout protection for the canal, which is only exposed externally. 

 The hyomandibular canal is 15 mm. long on the articular. 



Owing to an inward shelving on the part of the articular and pre-operculum, the next 

 10 mm. of the canal have no bony support, but are protected only by a ligamentous sheath. 

 This, of course, is developed in connection with the movement of the lower jaw. There 

 is also a strong upward curve on the part of the canal to meet the succeeding portion on 

 the pre-opei'culum. The seventh dermal tubule opens on to the canal at about the middle 

 of this ligamentous section. 



Having reached the pre-operculum, the hyomandibular canal lies at first somewhat 

 horizontally. It then takes a graceful curve upwards, so that the posterior portion of the 

 canal is practically vertical. Whilst on the pre-operculum the hyomandibular canal has 

 a length of about 80 mm. A thin bony lamella projects downwards anteriorly and 

 backwards posteriorly, forming an external lateral wall for the canal, but, however, 

 leaving it open ventrally in front and posteriorly behind. Commencing at about 11 mm. 

 from the posterior extremity of the canal, a ridge of bone 8 mm. long is seen to be 

 thrown up, which forms a posterior wall for the canal. At this region the anterior bony 

 lamina forming the external lateral wall of the canal tapers down, otherwise the 

 hyomandibular canal would in this region be completely enclosed in bone. The tube then 

 is here also incomplete externally, but is inore perfect than it is at any other part of the 

 pre-operculum. The hyomandibular canal ends blindly 10 mm. below the dorsal 

 extremity of the pre-operculum. The dermal tubules eight to twelve open on to the 



