4 CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



RECENT. 



Many characteristic structures of living Chimaeroids have been referred to as 

 indicating the primitive nature of the group. The following may be cited : 



Dentition and dermal defenses, by Jaekel (1901), who maintains that the dental 

 plates are primitive or "statodont," i. c., the ancestral condition of the "lyodont, " 

 or successional teeth of the later sharks. They have thus, if I understand 

 Jacket's view correctly, become greatly subdivided, so as to produce the cuspid 

 teeth of sharks. So, -top, the larger integumental plates of ancient Chimaeroids 

 are believed to have given rise to cuspid scales, and a somewhat similar view was 

 expressed by Pollard (1891). According to Schauinsland (1902), the scales of Callo- 

 rhynchus are of so primitive a nature as to be directly compared to those of the 

 earliest Silurian "sharks." Finally, Reis (1895) suggests that the curious unpaired 

 tooth of mesozoic Chimaeroids finds its homologue only in the ancient Acanthodia. 



Vertebral column, with delicate ring "vertebrae," characteristic of Chimseroids, 

 is, according to Schultze (1817), but the next stage above the notochordal con- 

 dition of the lamprey; to Hasse (1879) it represents a polyspondylous condition 

 ancestral to the diplospondyly of the simplest living sharks; to Gegenbaur (1901) 

 "less differentiated"; to Howes (1902) a purely "chordal type"; to Meyer (1886) 

 "possibly primitive"; to Rabl (1901) a column which has "not developed centra." 



Cranium and arches. According to Cope (1870), the autostylism of Chim- 

 aeroids is in itself primitive, in spite of the evidence of its secondary character, which 

 has been assumed on comparative anatomical grounds from the time of Johannes 

 Miiller (1838). So, too, Kitchen Parker (1883) inclines, though doubtfully, to its 

 primitive autostyly; and Gadow (1886) appears to have a similar view in stating 

 that dipnoans were descended from a "simple autostylic form." The curious 

 labial cartilages are regarded by Howes (1891) and others as homologous with those 

 of hag-fishes. And connected with these the levator anguli oris, according to Reis 

 (1896), suggests closely the condition in Acanthodian sharks. Allis (1898) also 

 suggests that the jaw muscle (adductor) is of a primitive type (i. e., interbranchial), 

 and in this he follows distinctly the more general conclusions of Vetter (1878), 

 which are, indeed, in the latest time confirmed by K. Fiirbringer. The second 

 branchial arch, it may here be mentioned, has been referred to several times 

 (v. infra] as retaining archaic features. The labial cartilages, furthermore, are 

 said to be primitive, inasmuch as they represent the most perfect condition of 

 preoral gill-arches known among recent gnathostomes (K. Fiirbringer, 1903, and 

 Schauinsland, 1903); and a presymphyseal cartilaginous element is regarded as a 

 primitive copula between the mandibular and a premandibular arch. In fact, the 

 entire series of copulae is archaic (Gegenbaur, 1901). 



Ribs are absent, a primitive character, according to Goeppert (1895). 



Fin structures are of peculiar interest. According to Jeffrey Parker (1886), 

 the Chimaeroid is the only vertebrate to retain rudiments of a third pair of limbs. Its 

 paired limbs furnish, according to Gegenbaur, M. Fiirbringer, and Braus, evidence 

 of the origin of the paired limbs from gill-arches. In this connection Howes (1886) 



