806 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



with the interspinous bones of the anal fin. The elements in 

 question formed of cartilage in the larva, become ossified in the 

 adult, and are known as the hypural bones. They may appear 

 in the form of a series of separate haemal arches, corresponding 

 in number with the primitive somites of this region, which 

 usually, however, atrophy in the adult, or more often are from 

 the first imperfectly segmented, and have in the adult the form 

 of two or three or even of a single broad bony plate. The 

 transitional forms between this state of things and that, for 

 instance, in Lepidosteus are so numerous, that there can be no 

 doubt that even the most peculiar forms of the hypural bones of 

 Teleostei are simply modified haemal arches. 



This view of the hypural bones is, moreover, supported by 

 cmbryological evidence, since Aug. Miiller 1 (p. 205) describes 

 their development in a manner which, if his statements are to be 

 trusted, leaves no doubt on this point. 



There are a considerable number of Fishes which are not 

 provided with an obvious caudal fin as distinct from the remain- 

 ing unpaired fins, i.e. Chimaera, Eels, and various Eel-like forms 

 amongst Teleostei, and the Dipnoi. Gegenbaur appears to hold 

 that these Fishes ought to be classed together in relation to the 

 structure of the caudal portion of their vertebral column, as he 

 says on p. 431 of his Comparative A natoiny (English Translation): 

 " In the Chimaerse, Dipnoi, and many Teleostei, the caudal 

 portion of the vertebral column ends by gradually diminishing in 

 size, but in most Fishes, &c." 



For our purpose it will, however, be advisable to treat them 

 separately. 



The tail of Chimaera appears to us to be simply a peculiar 

 modification of the typical Elasmobranch heterocercal tail, in 

 which the true ventral lobe of the caudal fin may be recognized 

 in the fin-fold immediately in front of the filamentous portion of 

 the tail. In the allied genus CallorJiynchus this feature is more 

 distinct. The filamentous portion of the tail of Chimaera con- 

 stitutes, according to the nomenclature adopted above, the true 

 dorsal lobe, and may be partially paralleled in the filamentous 

 dorsal lobe of the tail of the larval Lepidosteus (Plate 34, fig. 16). 



1 " Beobachtungen zur vergl. Anat. d. Wirbelsaule," Mailer's Archiv, 1853. 



