124 ME. F. J. COLE ON THE STRUCTUEE AND MOEPHOLOGY OF 



prominence, the ventral portion bearing a facet for the articulation of the second 

 sub-orbital. The sensory canal passes along the middle of the bone, marking it into two 

 almost equal halves. The dorsal border of the canal is slightly sigmoid, and anteriorly 

 it aj)proaches the border of the upper jaw, whilst the direction of the whole canal is 

 distinctly downward. A thin bony lamina (see below) projects downwards from above 

 and forms a roof and an external border for this portion of the infra-orbital canal, leaving 

 it open, however, ventrally. About 35 mm. from the posterior extremity of the lachrymal 

 this bony lamina is seen to be l)roken, thus forming a sort of notch and leaving this 

 portion of the infra-orbital canal wdthout any external boundary, and with only a portion 

 of its roof. The first four dermal tubules open into the iDfra-orbital canal on the 

 lachrymal, the first at the anterior extremity of the bone, and the tbird at the notch 

 mentioned above. 



The articulation of the lachrymal with the second sub-orbital is somewhat peculiar 

 and may here be descrilied. As mentioned above, and as is the case with the other 

 sub-orbitals, the lachrymal sends down externally a bony lamina the function of which is 

 to protect the external wall of the infra-orbital canal. With the posterior extremity of 

 tbe imperfect tube thus formed the dorsal portion of the anterior convex extremity of the 

 second sub-orbital becomes fitted or wedged in. Ventral to the external bony lamina 

 the second sub-orbital simply overlaps the lachrymal, and the large facet thus formed at 

 the posterior extremity of the latter bone measures 6 mm. antero-posteriorly. 



Second Sub-orbital. — The direction of this ossicle is backw^ards and downwards. Its 

 length dorsally at the sensory canal is about 20 mm., and its greatest width 22 mm. 

 Tlie anterior articular surface is convex, and is connected with the lachrymal in the way 

 above described. Posteriorly there is an oblique articulation with the third sub-orlntal. 

 The second sub-orbital is a large semi-cartilaginous plate, the dorsal border of which is 

 turned over so as to form a stout support for this portion of the infra-orbital canal. As, 

 however, the dorsal border does not fuse on to the large body of the plate ventral to the 

 sensory canal, the tube is imperfect ventrally, and is, in fact, only comj)leted by the 

 ligamentous sheath of the ossicle. The dermal tubules 5 and 6 open into the infra- 

 orbital canal on this ossicle — the former at the anterior extremity, where it overlaps tlie 

 lachrymal, and the latter near its posterior border. 



Third St(l)-orbital. — This ossicle continues the downward direction of its predecessor, 

 but is somewhat more horizontal. The infra-orbital canal is still passing backwards and 

 downwards. The length of the third snb-orl)ital is 15 mm., and its width 16 mm. The 

 anterior and posterior articular surfaces are oblique and roughly parallel, but the anterior 

 is much the longer. An imperfect tube for the lodgment of the sensory canal is formed 

 in the same way as in the second sub-orbital. The seventh dermal tubule opens into the 

 canal at the posterior extremity of the third sub-orbital. 



Fourth Sub-orbital. — The infra-orbital canal now begins to take an upward turn, and, 

 in consequence, the direction of this ossicle is upwards and backwards. It is further 

 midw^ay betw ecu the vertical and horizontal sub-orbitals, and measures 14 mm. in length 

 and 7 mm. in width. The sub-orbitals thus narrow from before backwards. The sensory 

 canal tube is formed similarly to those on the preceding sub-orbitals, but it must be noted 



