The Latero- Sensory Canals and Related Bones in Fishes. 495 



entire ectoderm and includes within it the ectodermal region that gives 

 origin to the plagiostomian canal. 



Both of these canals must be derived from some common earlier 

 condition of the system, which is apparently represented in the pit 

 lines of Amia. 



The plagiostomian canal is found not only in the Selachoidei and 

 Batoidei but probably also in the Ganoidei Chondrostei. 



The teleostean canal is found not only in all the Teleostei but 

 also in the Ganoidei Holostei, and quite undoubtedly also in the Crosso- 

 pterygii. 



In the Holocephela, the open latero- sensory grooves apparently 

 represent a condition antecedent to and hence in a way intermediate 

 to the plagiostomian and teleostean canals. 



In the Dipnoi, the manner of development of the canals is not 

 yet known. 



In the Plagiostomata the anterior end of the supraorbital canal 

 turns laterally and backward morphologically posterior to the nasal 

 apertures. In the Crossopterygii and Ganoidei Holostei it turns laterally 

 and backward in the same manner, but passes between the two nasal 

 apertures instead of posterior to them. In the Teleostei it runs for- 

 ward mesial to both apertures, and may even turn laterally and back- 

 ward anterior to them. These relations of this canal to the nasal 

 apertures are apparently without exception, and hence are of taxo- 

 nomic value. 



An ethmoidal, or anterior infraorbital cross -commissural canal is 

 found in the Ganoidei Holostei, the Crossopterygii, the Holocephala, 

 and in certain of the Ganoidei Chondrostei and Plagiostomata. It is 

 not known in any of the Teleostei but may be there represented by a 

 line of pit organs. In the Muraenidae there is a median ethmoidal 

 latero-sensory chamber formed by the coalescence of two parallel 

 longitudinal canals. This chamber is the homologue of the so-called 

 median canal of the Plagiostomata, and is a markedly selachian 

 characteristic. 



Spira cular latero-sensory organs are known only in the two living 

 Ganoidei Holostei and in certain of the Selachoidei. 



