494 Edward Phelps Allis jr., 



line to form a complete cross -commissure. The supratemporal and 

 supraorbital grooves would then be wanting in Pterichthys and Bo- 

 thriolepis, as they are in Homosteus, and the so-called median occipital 

 bone would necessarily contain parietal elements. The so-called post- 

 median bone would then probably be a median frontal, and the pre- 

 median bone an ethmoid. 



In the Labyrinthodonta, according to Fritsch [38], there were 

 "mucous canals", which are represented by half cylindrical grooves on 

 certain of the cranial bones, these grooves varying greatly in extent 

 and clearness, and becoming deeper and more clearly defined with age. 

 In the Stegocephali these grooves are said not to be found. The 

 character of the grooves and the varying relations of certain of them 

 to the cranial bones, seems to preclude them from being, all of them, 

 the exact homologues of the latero- sensory canals of fishes, and to 

 suggest that they may be the homologues of the grooves that lodge 

 the lines and patches of nerve sacs in Acipenser and Scaphirhynchus. 



Summary. 



The latero-sensory lines of fishes develop centrifugally from some 

 central point or points, following, in this development, pi-edetermined 

 lines of least resistance. These predetermined lines are remarkably 

 constant in position, but they nevertheless present certain variations, 

 certain of which are characteristic of different groups or orders of 

 fishes. A line of least resistance usually followed, in the centrifugal 

 development bj?^ a certain sensory line, may be, in certain other fishes, 

 invaded as it were, and so taken possession of by another sensory 

 line, this then becoming a characteristic of a group of fishes. 



The latero-sensory lines, once developed, may become enclosed in 

 canals, and, according to existing accounts, this enclosure takes place 

 in two distinctly different manners, which may be called the plagio- 

 stomian and the teleostean. 



The plagiostomian canal is the result of a dehiscence or delique- 

 scence in the central portion of a solid cord of cells formed by an in- 

 volution of the deeper layers only of the ectoderm, while the teleostean 

 canal is the result of a much larger involution which involves the 



