CHIM^ROIDS IN VERTEBRATE DESCENT. 1 55 



VI. MnsculatHrc. — The muscles of the branchial arches, like the arches 

 themselves, retain primitive features; thus the adductor of the jaw retains its inter- 

 branchial character. On the other hand, there is no ground for the belief that the 

 muscles of the shoulder girdle are unaltered; Gegenbaur (1901), for example, 

 frankly admits that the}' are more modified than those of sharks, and he calls 

 attention to the general blending of the segmental muscles of the trunk. There 

 appear also in Chimsera special muscles developed in connection with the erectile 

 spine and clasping organs which can best be interpreted as derived from the simpler 

 elements in sharks. 



VII. Nervous Sysfoii. — In this connection it may be remarked that some of 

 the primitive characters of Chimjera — open lateral line, separate nerve roots, 

 simple auditor}' organ — are clearly paralleled in sharks, e.g., Notidanids. 



SUMMARY. 



Chimseroids, accordingly, are widely modified rather than primitive forms. 

 The evidence contributed by anatomy, embryolog}', and paleontology is unmistak- 

 ably in favor of this interpretation. And there can be no doubt that the recent 

 forms retain less perfectly the general characters of the ancestral gnathostome than 

 do living sharks. On the other hand, it must be admitted that Chimceroids have 

 retained several characters of their Palaeozoic selachian ancestors which modern 

 sharks have lost. According to many converging lines of evidence we may indeed 

 go so far as to conclude that the ancestral Holocephali diverged from the selachian 

 stem near or even within the group of the Palaeozoic Cestracionts. * Indeed, the 

 recent Chimaeroids and Cestracionts retain many features of kinship. Among 

 these need only be mentioned at the present time approximations in dentition, 

 labial cartilages, articulation of mandibles, structures of fins, and urogenital system. 

 Even the com])licated egg-capsule of Chimaeroids hnds its nearest parallel in 

 the recent Cestraciont, a comparison often lost sight of on account of the spiral 

 arrangement of the lateral webs in the capsule of the latter form. 



From the standpoint of taxonomy, on the other hand, it must be clearly recog- 

 nized that the Chimaeroids have been separate from the early sharks for so long a 

 time and have acquired such different characters that they are to be given a high 

 rank among the divisions of the subclass Elasmobranchii, the equivalent, let us 

 sa}', of such groups as pleuracanths or pleuropterygians.f 



*This conclusion recalls the remarks of W. K. Parker, in his paper on the skull of cyclostomes (Phil. Trans. 1883, 

 p. 451) : "Even the Chimsroids come so near the ordinary Elasmobranchs as to suggest that their embryology would 

 not be so helpful (in the matter of the descent of the Cyclostomes) as one might imagine, especially if their solid upper 

 face has been acquired as a secondary modification and not a. primary condition, such as we see in the Tadpole, which 

 is especially solid and largely continuous with the basis cranii, in the larval Aglossal types, Dactylethra and Pipa. 

 (The interposition of those remarkable sharks, Cestracion and Notidanus, between the ordinary kinds and the Chimae- 

 roids, makes the likelihood of the solidity of the upper jaw being primary a very doubtful thing ; I once thought 

 otherwise, but found Mr. Balfour strongly set against me in this suggestion.)" 



t One recalls at this point an early remark of Huxley ; "For, considering, in addition to the cranial characters, 

 the structure of the vertebral column, and of the branchiae, the presence of an opercular covering to the gills, the 

 peculiar dentition, the almost undeveloped gastric division of the alimentary canal, the opening of the rectum quite 

 separately from and in front of the urogenital apertures, the relatively small and simple heart, the Chimaeroids are 

 far more definitely marked off from the Plagiostomes than the Teleostei are from the Ganoids." 



