128 CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



the forebrain, and the nasal region ; and it early develops conspicuous ridges which 

 evidently support and protect the eyes. And it is the latter organs, it may safely 

 be said, which have played the most important part in modifying the growth of the 

 cranium. For the orbital region is of enormous size, occupying no less than 50 per 

 cent of the entire length of the cranium;* and, correlated with this, between and 

 above the huge optic capsules, the growth of cartilaginous structures is retarded. 

 It follows, accordingly, that while the posterior and anterior parts of the chondro- 

 cranium are well developed, its mid- or orbital region is largely unformed, and this 

 is, I take it, the reason, the principal reason, that holocephaly has been developed, 

 to weld strongly together the anterior and posterior parts of the cranium where primi- 

 tively the orbital walls came to be suppressed as the eyes increased in size. Certain it is 

 that the wide palato-quadrate elements extend like firm beams between the anterior 

 and posterior moieties of the skull, and afford at the same time a support for 

 the great optic capsules. And in this result appears a suggestion why the palato- 

 quadrates appear so early and are so large in size; in fact, in no stage examined 

 has it yet been found that these palatine elements are altogether separate from the 

 cranium. In the earlier stages described (Callorhynchus) they are separate only for 

 about half their length, and from the details of that stage it is even doubtful whether 

 greater separateness ever occurs in the development of this element, earlier stages 

 showing probably a prochondrial continuum very much as one sees it in the 

 prochondrium of the paired fins of sharks. The skull of the Chimseroid, in a word, 

 is specialized even in early ontogeny ; witness, among other regards, the enormous 

 size of the posterior clinoid process, the huge fosse for the infundibulum, the exag- 

 gerated preorbital processes, the median frontal crest, and the interorbital vacuity. 



It is true, on the other hand, that certain skeletal structures in the chimaeroid 

 head retain a primitive character possibly because they have been spared func- 

 tional changes by the very fact that the palato-quadrate element has fused with 

 the cranium. As primitive features we may here mention: (i) The perfect 

 condition of the copulae of the branchial arches. (2) The presence of a pha- 

 ryngeal element in the hyoid arch which resembles the pharyngobranchials of the 

 hinder arches. (3) The relatively large and discrete labial cartilages, as probable 

 premandibular arches, and finally (4) the presence of a symphyseal cartilage as 

 (Schauinsland, K. Fiirbringer) the probable serial homologue of a basihyal. These 

 characters are expressed, slightly schematized, in fig. in, and may be compared 

 with the corresponding structures in sharks (fig. no). In these figures serially 

 homologous parts are indicated by shaded or unshaded areas. 



It should be mentioned, in passing, that even the branchial region of Chimaeroid, 

 in spite of the foregoing primitive characters, is not without convincing evidence 

 of precocious specialization witness the early appearance of the supporting extra- 

 branchials of the hyoid arch, which are prophetic of the opercular flap of the adult. 



The problem of rostral cartilages receives no evident solution in Chimaeroid 

 development. The anterior azygous process of selachians, which rises from the 



* In the skull of the shark (f. g., Scyllium) at a corresponding stage the orbit occupies about 30 per cent of the 

 entire length of the cranium. 



