The Latero-Sensory Canals and Belated Bones in Fishes. 475 



In Chimaera, as just above stated, the sensory lines all lie in open 

 dermal grooves, an enclosed canal nowhere being found; and the walls 

 of these grooves are said to be supported, opposite the enclosed sense 

 organs, by a series of narrow half rings which are not of true bone» 

 but of a bony material that is called by Solger "osteoides Gewebe". 

 In the generi cally related Callorynchus, the sensory lines are said by 

 Solger [67, p. 96] to also lie in open grooves, but Garman, in a more 

 recent work [39, p. 75], says that this fish differs from Chimaera "in 

 possessing canals that are tubes instead of furrows". In the fossil 

 Chimaeroid Ischyodus, also, the sensory lines would seem to have 

 been enclosed in canals, judging from Zittel's figures [83, p. Ill] 

 of this fish and his reference to the supporting "rings of the slime- 

 canals". 



In the Batoidei the sensory lines are, according to Garman [39], 

 always enclosed in tubes. In the Selachoidei they are, according to 

 the same author, generally so enclosed, being found as dermal grooves 

 only "on species of Heptabranchias and on Chlamydoselachus". 

 Solger [6?] however says that they are also found as grooves in the 

 tail region of Echinorhinus spinosus, and Woodward [81] says that 

 they existed as grooves in the fossil Liassic selachian, Squaloraja. The 

 grooves of this latter fish are also said by AVoodward to have been 

 supported by incomplete rings exactly similar to those of Chimaera. In 

 living selachians, it is to be noted, the walls of the lateral canals are 

 said to be simply of tough connective or flbro-cartilaginous tissue, with- 

 out bony support of any kind. 



The canals of plagiostomes are, I believe, always considered to be 

 strictly homologous with the canals of teleosts, and Balfour [13, vol. 2, 

 p. 445] considered the closed canals of both these groups of fishes as 

 secondary, as compared with the open grooves of Chimaei a. But are 

 the canals in these two great divisions of fishes homologous? 



In plagiostomes the lateral canal develops, according to Balfour 

 [13, p. 444], "by the formation of a cavity between the mucous and 

 epidermic layers of the epiblast, and the subsequent enclosure of this 

 cavity by the modified cells of the mucous layer of the epiblast which 

 constitute the lateral line". According to Solger [68, p. 464] the 



