The Latero-Sensory Canals and Related Bones in Fishes. 487 



I of my descriptions of Polyodon, either with or without certain of 

 the immediately posterior latero-sensory ossicles, is quite certainly the 

 squamosal, and not the postfrontal bone, this latter bone then being 

 represented in my bone 2. Bone B^ would then probably be the 

 parietal, as Bridge concluded, and 5^ a part, but not all of the frontal, 

 the remaining part of the latter bone being represented in bone 3 of 

 my descriptions. 



Fossil fishes. 



From the proceeding descriptions it is evident that the latero- 

 sensory canals of fishes, when properly known and understood, give 

 very important, and often positive evidence as to the homologies of 

 the bones they traverse. This has led me to apply the test of these 

 canals to the cranial bones of the early ganoids and dipnoids (follow- 

 ing Zittel's classification [83, p. 56]), these fishes being, in the litera- 

 ture at my disposal, the only fossil fishes of which I can find servi- 

 ceable descriptions. But the attempt to compare the canals and 

 related bones of these fishes with those of living fishes is, in every 

 instance, because of the want of definite details, a purely tentative 

 process. 



In Dipterus, according to Pander [56, p. 7], "Der grösste Teil der 

 Schuppen des Schädels ist mit Poren besetzt, die wahrscheinlich die nach 

 aussen sich öffnenden Kanäle des Seitenporensystems darstellen. Am 

 stärksten ist dieses System an den Seitenplatten des Kopfes entwickelt, 

 während die in der Mittellinie liegenden gewöhnlich glatt und eben 

 sind". The large number of these pores certainly indicates a highly 

 developed condition of the latero-sensory canals, and their distribution, as 

 shown in Pander's figures, would seem to indicate that the canals from 

 which they arise must have a general course not unlike that in Cera- 

 todus [78]. And as the canals of Ceratodus would seem to be the homo- 

 logues of those in Acipenser, the bones traversed by the canals, in 

 Dipterus, would then bear some relation to those traversed by the corre- 

 sponding canals in Acipenser. But as the bones of Acipenser are quite 

 probably not yet properly homologised with those of teleosts, it is 

 evidently useless to attempt to homologize those of Dipterus. All that 



