EVIDENCE OF PRIMITIVE CHARACTERS. 5 



maintains that the paired fins of Chima-ra are ancestral to those of sharks and 

 dipnoans. Rabl (1901) also refers, but in a dififerent aspect, to the primitive nature 

 of the fins of Chima^ra. By several writers the unpaired fins are regarded as 

 primitive. The fin spine, as Reis (1896) maintains, shows the granular calcification 

 of the mesozoic Ischyodus. The mixipterygia are "of less compound construc- 

 tion" (Jungersen, i8gS) than those of sharks. 



Brain, nerves, and sense organs have received considerable attention. \'alentin 

 (1842) states that in its brain Chimjera is intermediate between cyclostomes and 

 plagiostomes, and his view is shared, more or less distinctly, by Johannes Miiller, 

 Mikloucho-Macleay.Gegenbaur, Wilder, and M. b^iirbringer. To Burckhardt(i893) 

 the Chimjeroid brain suggests characters allied on the one hand to the primitive 

 sharks, on the other to the lower ganoids, and according to Studnicka (1895) the 

 forebrain is nearer the primitive form of the selachian brain than even that of 

 Notidanid. Jaekel (1902) holds also that in Chimrera, alone among fishes, there 

 appears an epiphy.seal opening in the cranial roof. In the matter of cranial nerves 

 Cole (1896) states that " Chimsera is unrivalled among vertebrates, first, for the ease 

 with which its nerves may be dissected and, second, for the almost ideal results 

 that may be attained," as well as for the peculiarity of indei>endent nerve roots, 

 "archaic and perhaps i)rimitive in type." Similarly, b'urbringer (1897) comments 

 upon the peculiar conditions of the nerves of the occiput. Collinge (1896) notes 

 also the simiilicity of the mucous-canal system, which, hc' believes, separates widely 

 Chimseroids and sharks. iMom the standpoint of the auditory organ Retzius (i 88.| ) 

 places Chimseroids in the ancestral line of the modern elasmobranchs. Gegenbaur 

 (1901), finally, notes that the fiattened cord is jirimitive, like that of cyclostomes. 



Viseeral />een//an7/es have also been given considerable notice. Thus Huxley 

 (1872) refers to the "almost undeveloped gastric division of the alimentary canal, 

 [and] the relatively small and simple heart." Gegenbaur (1901 ) is inclined to regard 

 the few turns of the spiral intestinal valve as the ancestral condition of the gut of 

 Lepidosteus and Ceratodus. Leydig(i85 1), followed by Mazza and Perugia (i 894), 

 suggests that the many small brown glands of the rectum represent the ancestral 

 condition of the digitiform appendix of sharks. Redeke(i899) maintains that in the 

 structure of the kidney Chimaeroids are primitive, since, among other features, they 

 retain a remarkable metamerism and have not the modified ( ieschlechtsniere of sharks. 



The foregoing are the i)rincii)al lines of argument in favor of the primitive 

 position of Chimaeroids. Whether they can be maintained in the light of additional 

 evidence, notably on the side of eml)ryology, is a cpieslion which will be discussed 

 in the present memoir. 



To summarize the i)roblem : Are the Chim?eroid fishes the least modified 

 descendants of the primitive gnathostome? Or are they, on the contrary, degen- 

 erate, specialized, or widely modified? Are they, in other words, close to ancestral 

 forms which gave rise to sharks, with which they are obviously associated — or are 

 they but modifications of the shark-like form? In spite of the formidable list 



