108 STUDIES ON THE ELASMOBRANCH SKELETON, 



is a much smaller lamella which develops lateral projections 

 embracing the dorsal extremities of the pectoral arch, so as to form 

 sockets for the reception of the latter. 



There are about twenty-five pairs of rather long slender ribs. 



Skull and Visceral Arches. (Plate II., figs. 1 and 2.) 



In general form the skull is somewhat expanded laterally and 

 depressed dorso-ventrally, distinguished by its elongate rostrum 

 and large olfactory capsules. 



The plane of the foramen magnum slopes slightly forwards. 

 Below it is the deep mesial excavation for the reception of the 

 odontoid process, the concave articular surface for which is nearly 

 as wide as the foramen. The condyles are borne on short lateral 

 processes. The vagus foramen is separated by a considerable 

 interval from the foramen magnum. Immediately in front of and 

 external to it is a slender bridge of cartilage. 



The articular surface for the hyo-mandibular is long and 

 narrow, placed, as in Hypnos, close to the base of the skull and 

 extending on to the postero-lateral process. The auditory 

 foramina are very large, and are placed well apart from one 

 another, only a very slight depression connecting them.* The 

 elevations marking the position of the anterior and posterior 

 semi-circular canals are very well-marked. Behind the orbit is a 

 distinct post-orbital process, and in front of it are the very large and 

 laterally pi'ojecting olfactory capsules. As in the Batoidei generally 

 theie is no basal plate. The foramen for the trigeminal is 

 separated from that for the facial by a slight interval ; behind the 

 latter as in Rhynchobatus is an oblicpiely-placed bridge of 

 cartilage. With the outer and posterior angle of the olfactoiy 

 capsules is connected a compressed, curved, pointed cartilage, which 

 represents the lateral process of the ethmoidal region of 

 Rhynchobatus. 



* The absence of a deep parietal groove seems to he characteristic of the Batoidei. 



