76 STUDIES ON THE ELASMOBRANCH SKELETON, 



In most Sharks the orbital cavity is bounded below by a plate of 

 cartilage— the basilar plate ; but this is absent in all the Rays. It 

 is frequently bounded in front and behind by prse- and post-orbital 

 processes ; and its wall is perforated by apertures for the egress of 

 the facial, trigeminal, abducent and oculo-motor nerves. A shallow 

 groove passing forwards from the aperture for the trigeminal marks 

 the coui'se of the ophthalmic branch of that nerve, and a series of 

 foramina in the roof indicate the points at which the supra-orbital 

 branches of the same nerve penetrate towai'ds the roof of the skull. 

 In front near the upper border of the orbit is a canal or notch 

 by which the ophthalmic nerve reaches the upper surface of the 

 skull ; and in front of the orbit is a canal or notch (the ethmoidal 

 canal or notch), by which it again passes downwards towards the 

 lower lateral region of the olfactory capsule. 



In front of the orbits ai*e the olfactory capsules, which are more 

 or less completely enclosed in cartilage, and are usually solid^ 

 connected with the rest of the cranium. Related to the olfactory 

 aperture is a small cartilage, the olfactory cartilage usually of the 

 form of an incomplete ring. 



Between the nasal capsules the mesial portion of the skull is 

 usually produced forwards into a longer or shorter rostrum, which 

 may be single and contain a canal continuous with the cranial 

 cavity, or may be composed of three solid bars of cartilage, a 

 mesial and two lateral, which coalesce anteriorly. Directly or 

 indirectly related to the skull are the palato-quadrate, Meckelian> 

 hyoid and branchial arches. 



At the sides of the gape are the labial cartilages, of which there 

 are usually two pairs above and one below. In the Rays labial 

 cartilages, as a rule, are absent. 



The upper and lower jaws of the Elasmobranchii consist of the 

 palato-quadrate and Meckelian cartilages respectively, the sub- 

 stance of the cartilage being usually deeply impregnated with 

 osseous matter. The palato-quadrate is distinguishable in the 

 Sharks into an anterior, palatine, and a posterior, quadrate, 

 portion. It presents in Notidanus a process for articulation with 

 the post-orbital process. In the Notidanidce the palatine cartilages 



