128 MB. F. J. COLE ON THE STEUCTURE AND MORPHOLOGY OF 



Third Supra-temporal or Extra-scapula. — The third supra-temporal is a somewhat 

 long bone supporting tlie anterior extremity of the lateral canal as it passes downwards 

 and backwards from the pterotic. Its greatest length is 22 mm., and its greatest width 

 9 mm. It consists of a plate slightly convex intei*nally, the dorsal edge of which is 

 turned over, but not very markedly, to form an imperfect tube. Its ventral border is 

 quite straight, whilst its dorsal border is somewhat irregular. Anteriorly it articulates 

 by means of an obliqvie and well-marked concavity with the pterotic, the dorsal border of 

 the third supra-temporal being prolonged forwards so as to fit into a corresponding 

 shelving on the pterotic. Ventrally the latter presents a large triangular facet 10 mm. 

 in length from base to apex, on w^hich the ventral portion of the anterior extremity of 

 the third supra-temporal rests and articulates. This articulation is very compact and 

 strong. Posteriorly the articulation is with two ossicles : a, by a long dorsal and 

 slightly concave facet with the second supra-temporal ; b, by an oblique concavo-convex 

 facet with the fourth supra-temporal. The third dermal tubule opened into the lateral 

 canal near the posterior extremity of the third supra-temporal, and slightly posterior to 

 the entry of the supra-temporal canal into the lateral. This tubule was present on the 

 left side of the fish, but not on the right. 



Fourth Supra-temporal or Extra-scapula. — This consists of a flat internal plate 11 mm. 

 across at its widest part, and with a maximum length of 16 mm. Its dorsal edge is 

 turned over so as to form an external jirocess protecting the sensory canal. Anteriorly 

 it has a close ligamentous articulation with two ossicles : «, by a convexo-concave ventral 

 facet with the third supra-temporal ; h, by a slightly convex dorsal facet with the second 

 supra-temporal. The latter facet is internal to the first and passes back at an angle 

 to it. Posteriorly the fourth supra-temporal rests on and overlaps the angle formed by 

 the two forks of the post-temporal. The fourth dermal tubule opens into the lateral 

 canal near the centre of the fourth supra-temporal. 



Post-temporal or Supra-scapula *. — The angle formed by the union of the two limbs of 

 the post-temporal now comes to the surface of the skull and supports the next 9 mm. of 

 the lateral canal. It is here that the lateral canal commences to approacli the surface 

 of the body and to take tip that position in which it is to be found immediately under 

 the skin. The fifth dermal tubule opens just at the posterior extremity of the post- 

 temporal, and almost between that bono and the first lateral line ossicle. 



Behind the post-temporal the lateral canal loses the support of the bones of the skull 

 and pectoral girdle, and is only protected at intervals by the lateral line ossicles, which 

 themselves diminish posteriorly as the canal approaches the skin and decreases in size. 



At about the middle of the fish the lateral canal lies immediately under the skin, and 

 is situated at the side of the body, its position being indicated by the modified scales 

 lying immediately over the canal. In the specimen from which the above description 

 has been written, it was about 55 mm. ventral to the anterior extremity of the dorsal fin. 

 It is supported behind the post-temporal by a series of imperfect bony cylinders, the 



* The suin'a-elavicle of some authors (t'. (/. McMurrich, 136, and Gegeubaur). It is imiiossible, however, to 

 homologiso the boues usually termed post-temporal and supra-clavicle. 



