HOLOCEPHALI. 157 



plates with cutting edges. (There are two pairs in the upper 

 and one pair in the lower jaw. ) The intestine has a spiral 

 valve and the anus isjnjfront of the urogenital aperture. There 

 is a conus arteriosus with three rows of valves. 



The brain is characterised by the great length of the thala- 

 mencephalon, and the cerebrum is very small. The olfactory 

 peduncles are long and there is an optic chiasma. The pineal body 

 is as hi Plagiostomes, and there is an extracranial part of the 

 pituitary body lodged in a pit on the base of the skull. The 

 cranial nerves* are arranged in the usual manner ; the roots of 

 the fifth and seventh are more distinct than in most fishes. 



There is a pericardio-peritoneal canal. 



The urogenital organs appear to be similar to those of Plagio- 

 stomes. In the female the shell gland is large. In the male 

 there is a large vesicula seminalis, and the miillerian ducts are 

 complete tubes opening at each end. 



Thex-are, oviparous andJiayjpi la,rge_eggs. The eggshell is 

 covered with hair-like processes, and may attain a great size. 

 They are probably laid in deep water, where the young are for 

 the most part found. 



They have existed since the Jurassic period. 



In many points of structure these animals depart from other 

 Elasmobranchs, and they have by some authorities been removed 

 from that group and raised to the rank of a sub-class with 

 affinities to the Dipnoi by the characters of their skull and teeth. 

 There is much to be said for this view, for they present affinities 

 to more than one piscine sub-class : to Elasmobranchs by their 

 placoid scales, cartilaginous skeleton, absence of membrane bones, 

 their gill-laminae, the open otocyst, the ampullary canals, the 

 form of the brain, the structure of the urinogenital organs, their 

 fin skeleton and claspers, and by their large eggs and develop- 

 ment : to the Ganoids by the separation between the urinogenital 

 sinus, and the alimentary canal, and by the incomplete internal 

 wall of the auditory capsule : and to the Dipnoi by the last- 

 named feature, by their autostylic skull, their peculiar teeth, and 

 their vertebral column. They differ from Elasmobranchs and 

 resemble Ganoids and Dipnoi in having an operculum, but they 

 stand by themselves in having only four branchial clefts and a 



* F. J. Cole, Cranial Nerves of Chimaera. Trans. Eoy. Soc. Edinburgh, 

 38, 1896, p. 631. 



