112 STUDIES ON THE ELASMOBRANCH SKELETON, 



SELACHOIDEI. 



In the skull the post-orbital processes are usually well developed, 

 the orbit is usually provided with a cartilaginous floor formed of 

 the basal plate ; there is always a palato-basal articulation ; the 

 rostrum usually consists of three bars with large foramina at the 

 base. There are a series of external branchial arches ; the first 

 branchial arch never articulates with the skull ; the hyoid arch is 

 supported by the hyo-mandibular ; the copula of the hyoid has the 

 form of a broad plate connected with its distal extremity. The 

 pectoral fin is not connected with the skull by means of an ant- 

 orbital cartilage ; the ventral portion of the pectoral arch is 

 divided in the middle by a more flexible portion into two lateral 

 halves usually slightly movable on each other, and the dorsal 

 extremities do not articulate with the spinal column. The pro- 

 and meta-pteiygia of the pectoral fin are never greatly elongated 

 and usually have the form of relatively broad plates. 



PALuEOSELACHII. 



(NOTIDANIDiE.) 



The vertebral column is scarcely ossified. There are two 

 neural arches for each centrum, at least in the caudal region. 



The occipital region of the skull is not so sharply marked off 

 from the spinal column as in other Elasmobranchii ; it presents 

 above a mesial ridge continuous with the spinous processes, and at 

 the sides riclges continuous with the line of the transverse 

 processes. The plane of the occipital region is vertical or inclined 

 from below upwards and backwards. There is no lateral occipito- 

 vertebral articulation. The principal vagus foramen is placed far 

 from the foramen magnum ; the lower roots of the nerve pass out 

 by from three to five distinct canals which are in line with the 

 foramina for the spinal nerves. The vestibulum forms a distinct 

 eminence on the surface of the infero-lateral portion of the 

 auditory region. The articular surface for the hyo-mandibular is 

 simple and not sharply marked off from surrounding parts. The 

 post-oi"bital process presents an articular surlace for the palato- 

 quadrate. The orbit has no cartilaginous floor. There is an 



