474 FISHES CHAP. XVII 
infer that these Fishes breed in deep water. As might be 
expected, little is known of the embryology of any of the Holo- 
cephali, but that ttle 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 Elasmobranchs. 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.' ‘ 
The Chimaeridae first appear in the Lower Oolites, and attain 
their maximum development in the Cretaceous and the Eocene.” 
Ganodus is an Oolitic genus. IJschyodus ranges from the Lower 
Oolites to the Lower Cretaceous. Hdaphodon is Cretaceous and 
Kocene, extending, however, into the Miocene, and Hlasmodus 
ranges from the Upper Cretaceous into the Kocene. Teeth of 
the existing genus Callorhynchus occur in the Cretaceous of 
New Zealand, and of Chimaera in the Upper Tertiary of 
Kurope 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. 
' Bashford Dean, Mem. New York Acad. Sci. ii. Pt. i. 1899, p. 28; Biol. Bull. 
iv. 1903, p. 270. 
2 EK. T. Newton, Mem. Geol. Surv. Monogr. iv. 1878; Riess, Palaeontogr. 
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. 
