BY WILLIAM A. HASWELL, M.A., B.SC. 113 



ethmoidal canal. There is no tri-radiate rostrum. Representing 

 the ant-orbital cartilage of Rays is an ant-orbital process. There 

 are either six or seven branchial arches ; the external arches are 

 incomplete. There is only one dorsal fin ; its rays are supported 

 by a broad basal cartilage. 



NE0SELACH1I. 



The centra of the vertebrae are well ossified. 



The occipital region is well marked off from the vertebral 

 column. The plane of the foramen magnum is vertical or slopes 

 forwards. The principal vagus foramen is usually approximated 

 to the foramen magnum, and there is never a row of accessory 

 foramina in line with the foramina for the spinal nerves. There is 

 no distinct elevation on the surface of the skull marking the 

 position of the vestibule. The articular surface for the hyo- 

 mandibular is complex. The post-orbital process never presents 

 an articular surface for the palato-quadrate. The orbit has a 

 cartilaginous floor. There is no process representing the ant- 

 orbital cartilage of the Rays. There are never more than five 

 branchial arches. There are two dorsal fins which may or may not 

 present broad basal cartilages. 



Cestraciontid^e. 



The centra of the vertebra? present radiating bony lamellae. 



There is only a small tubercle representing the occipital crest, 

 and the occipital region is sharply marked off from the vertebral 

 column. The principal vagus opening is placed close to the lateral 

 occipital process, and there is no row of accessory foramina. 

 The elevation marking the position of the posterior semi-circular 

 canal is placed on the posterior aspect of the cranium ; there is no 

 distinct elevation for the vestibule. There is an ethmoidal canal. 

 The auditory foramina open at the bottom of a fossa which is 

 continued forwards into a groove excavated on a longitudinal 

 mesial ridge. The orbit has a cartilaginous roof. There is no 

 prominent rostrum ; the ethmoidal region is longer than in other 

 8 



