The Holocephali, or Chimaeras 563 



Relationship of Chimaeras. As to the origin of the Chimaeras 

 and their relation to the sharks, Dr. Dean has this recent ("The 

 Devonian Lamprey") and interesting wofd: 



"The Holocephali have always been a doubtful group, 

 anatomy and palaeontology contributing but imperfect evidence 

 as to their position in the gnathostome phylum. Their em- 

 bryology, however, is still undescribed, except in a brief note 

 by T. J. Parker, and it is reasonably looked to to contribute 

 evidence as to their line of descent. The problem of the relation- 

 ships of the Chimaeroids has long been of especial interest to 

 me, and it has led me to obtain embryonic material of a Pacific 

 species of one of these forms. It may be of interest in this 

 connection to state that the embryology of this form gives 

 the clearest' evidence that the wide separation of the Selachii 

 and Holocephali is not tenable. The entire plan of develop- 

 ment in Chim&ra colliei is clearly like that of a shark. The 

 ovulation is closely like that of certain of the rays and sharks: 

 the. eggs are large, the segmentation is distinctly shark-like; 

 the circular blastoderm overgrows the yolk in an elasmobranchian 

 manner. The early embryos are shark-like; and the later 

 ones have, as T. J. Parker has shown, external gills, and I note 

 further that these arise, precisely as in shark-embryos, from the 

 posterior margin of the gill-bar. A spiracle also is present. 

 A further and most interesting developmental feature is the 

 fact that the autostylism in Chim&ra is purely of secondary 

 nature and is at the most of ordinal value. It is found that 

 in a larva of Chim&ra measuring 45 mm. in length, the 

 palato-quadrate cartilage is still separated from the skull by 

 a wide fissure. This becomes gradually reduced by the con- 

 fluence of the palato-quadrate cartilage with the skull, the 

 fusion taking place at both the anterior and posterior ends of 

 the mesal rim of the cartilage. The remains of the fissure are 

 still well marked in the young Chinxzra, four inches in length; 

 and a rudiment of it is present in the adult skull as a passage- 

 way for a nerve. Regarding the dentition: it may also be 

 noted in the present connection that the growth of the dental 

 plates in Chim&ra suggests distinctly elasmobranchian con- 

 ditions. Thus on the roof of the mouth the palatine plates 

 are early represented by a series of small more or less conical 



