4/4 FISHES CHAP, xvn 



infer that these Fishes breed iu deep water. As might be 

 expected, little is known of the embryology of any of the Holo- 

 cephali, but that little adds further proof of the Elasmobranch 

 relationship of the group. The segmentation of the egg of 

 Chimaera and the overgrowth of the yolk by a circular blasto- 

 derm are essentially as in Elasmobrauchs. The early embryos 

 are said to be shark-like, and to possess both spiracles and 

 " external gills," and the primary upper jaw is less completely 

 confluent with the skull than in the adult. It is also said that 

 the palatine dental plates are represented at an early stage by 

 series of small, more or less conical elements, which, outwardly at 

 least, resemble the rudiments of the grinding teeth of the 

 Cestraciont Sharks. 1 



The Chimaeridae first appear in the Lower Oolites, and attain 

 their maximum development in the Cretaceous and the Eocene. 2 

 Ganodus is an Oolitic genus. Ischyodus ranges from the Lower 

 Oolites to the Lower Cretaceous. Udaphodon is Cretaceous and 

 Eocene, extending, however, into the Miocene, and Masmodus 

 ranges from the Upper Cretaceous into the Eocene. Teeth of 

 the existing genus Callorhynchus occur in the Cretaceous of 

 New Zealand, and of Chimaera in the Upper Tertiary of 

 Europe and Java. The fossil Holocephali afford little evidence 

 of the origin of the group from more typical or more primitive 

 Elasmobranchs. So far as their structure is known, they all 

 possess the essentially distinctive features of their modern repre- 

 sentatives, and offer little evidence of transitional forms. The 

 surviving Chimaeroids seem to have acquired a more specialised 

 dentition, but in other respects they are either more primitive, 

 or possibly somewhat degenerate. 



1 Bashford Dean, Mem. New York Acad. Sci. ii. Pt. i. 1899. p. 28 ; Biol. Bull. 

 iv. 1903, p. 270. 



2 E. T. Newton, Mem. Geol. Surv. Monogr. iv. 1878 ; Riess, Palacontogr. 

 xxxiv. 1887, p. 1 ; Smith Woodward, Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 52 ; 

 Zittel, Text-Book of Palaeontology, English ed., London and New York, ii. 1902, 

 p. 46. 



