HOLOCEPHALI. 24! 



since a membranous fold of the skin, the operculum, grows 

 backwards over them, so that but a single aperture is visible 

 exteriorly. The spiracle is closed. The pelvic fins are abdomi- 

 nal in position and bear claspers in the males, while in front of 

 these fins there is a pit containing an anterior clasper armed 

 with hook-like spines, the function of which is unknown. In 

 addition there is borne on the head in Chimcera and Callor- 

 hyncJius a peculiar frontal clasper differing in the sexes. The 

 skin in recent species is naked, and the lateral line on the trunk 

 is an open groove. The tail is heterocercal. 



The mouth is terminal, the nostrils dorsal, the latter not 

 communicating with the mouth. The teeth are in the form 

 of strong plates, two pairs in the upper jaw, a single pair in 

 the lower. These have areas (tritors) specially hardened by 

 deposits of lime. The alimentary canal is almost straight ; the 

 intestine possesses a spiral valve. No cloaca is present, the 

 urogenital ducts opening behind the vent. The gills are re- 

 lated to the septa as in the selachians. No air-bladder is 

 developed. 



The brain has the hemispheres distinct from each other, the 

 olfactory lobes being separated from the cerebrum by long 

 olfactory tracts. The twixt brain is extremely long ; an optic 

 chiasma occurs. The heart is like that of selachians, three 

 rows of valves being present in the conus. 



The skeleton is cartilaginous. The notochordal sheath con- 

 tains cartilage rings more numerous than the segments ; and 

 the neural tube, composed of neural arches and intercalaria, is 

 very high. The cranium is high and narrow, and the pterygo- 

 quadrate is indistinguishably fused with the cranium (autosty- 

 lic). The skull is movably articulated to the vertebral column 

 by a (basi) occipital condyle. The shoulder girdle is like that 

 of elasmobranchs ; but the pelvic girdle consists of right and left 

 halves, connected by ligament. The excretory organs and fe- 

 male reproductive organs are much as in other elasmobranchs ; 

 the male organs are noticeable for the large size of the epi- 

 didymes and the seminal vesicles developed from the posterior 

 portions of the vasa deferentia. The Miillerian ducts also re- 

 tain their lumen in the male, and connect with the ccelom by 



