On certain featnres of the lateral canals and cranial bones of Polyodon folium. QQ\ 



of such a secondary enclosing of a surface organ in the mouth of a 

 primary tube of a lateral canal. It seems to me, however, that Col- 

 linge's fig. 4, which purports to be a section through a cluster pore, 

 is, on the contrary, a section through a primitive poro; while fig. 5, 

 instead of being a section through one of the latter pores, is probably 

 a section through a distinctly different organ, doubtless one strictly 

 similar to the surface pit-organs of Amia, and which one might naturally 

 except to find on the head of this fish. 



OOO o 



B 



The so-called primitive pores are found in distinct and separate 

 groups, many of these groups being subdivided, more or less distinctly, 

 into separate subgroups. Cut A shows one of these groups, taken 

 from the dorsal surface of the head of one of my specimens in a 

 place where the epidermal layer of the skin was still in place but 

 much disintegrated. Cut B shows another group, taken from the hyo- 

 mandibular region and selected because of the intimate relation of 

 the group to certain of the ""cluster pores" of the hyomandibular canal. 

 The epidermal layer of the skin was here entirely gone, but the figure 

 shows, beyond question, that the relation of the cluster and primitive 

 pores to each other is one of proximity only. The two figures also 

 seem to indicate, and sections through other groups seem to almost 

 positively confirm, that the several organs of a group of primitive 

 pores all arise by the subdivision of a single patch of sensory tissue 

 which primarily represents the group, this subdivision being brought 

 about, not by a process of budding, as in the lateral canal organs of 

 Amia, but by the gradual growth of ridges, formed of the underlying 

 tissues, which force their way, wedge-like, toward the surface, through 

 the overlying sensory tissue. Certain of these ridges are certainly 

 bisecting ones, each of which cuts a pre-existing organ into two parts. 

 But many of them would seem to develope at the same time, cutting 

 the pre-existing organ or patch at once into several parts. As the 



