478 Edward Phelps Allis jr., 



llie sensory tissue condenses into a solid connecting strand, and this 

 strand and the connected organs would seem to be the homologue of 

 the entire canal of plagiostomes. The strand and organs then un- 

 questionably become involved in a larger involution similar to that de- 

 scribed by me in Amia, destined to give rise to the lateral canals of 

 the adult, but Wilson does not describe this involution. 



The canals of teleosts and plagiostomes are thus certainly not 

 homologous structures, and neither could be developed directly from the 

 other. Both must accordingly be derived from some earlier condition, 

 which would seem, from Coggi's researches, to be represented in the 

 pit lines of Amia and teleosts. In Chimaera there apparently is, as 

 already stated, a somewhat intermediate condition; for, on the trunk 

 of this fish, the small sensory grooves apparently belong to the plagio- 

 stomian type, while on the head the much larger grooves are appa- 

 rently in process of enclosure in the teleostean manner. The two 

 types of canal each apparently develop in relation to strictly homo- 

 logous sensory lines, but the points of origin of the primary tubes 

 being so different in the two types — in one arising opposite the 

 sense organs and in the other between the organs — the points of ana- 

 stomosis of the canals may not be strictly homologous. The develop- 

 ment of the canals in the plagiostomes is much too little known to 

 permit any opinion on this and also on many other questions of im- 

 portance in this connection. 



Ganoidei Chondrostei. 

 Acipenser sturio. Van Wijhe [78], in 1882, briefly described the 

 canals in this fish. He says that the main infraorbital canal traverses 

 in succession the suprascapular, lateral supratemporal (exoccipitale 

 externum, Gegenbaur: epiotic Huxley), and squamosal bones, and then, 

 in the middle of this latter bone, turns downward and traverses in 

 succession the dermal postfrontal and infraorbital bones. In the squa- 

 mosal, the canal is said to anastomose with the supraorbital canal, 

 which latter canal traverses the frontal and then the nasal. A supra- 

 temporal cross-commissure is said to traverse the three supratemporal 

 bones of the fish, two of which bones are the lateral supratemporals 



