On certain features of the lateral canals and cranial bones of Polyodon folium. 665 



says, the canal does not, on leaving the most anterior ossicle of the 

 lateral line, immediately enter bone C^ of Bridge's descriptions. 

 Before reaching that bone it first traverses a short canal in a bone 

 that is considered by Gegenbaur (9, p. 113) as the most dorsal one 

 of two supraclavicularia, the canal traversing that bone obliquely, 

 near its dorsal end, exactly as it traverses the supraclavicular in 

 Amia. This relation of this bone, in these two fishes, to a lateral 

 canal, as well as its general relations, in each of them, to the skull 

 and to the other bones of the shoulder girdle, all show conclusively 

 that the bones of the two fishes are homologous. In Polyodon the 

 bone is said by Gegenbaur (9, p. 106) to be a "Deckknochen", pri- 

 marily developed in relation to an underlying cartilage, this cartilage 

 having wholly disappeared in Polyodon but being still retained in 

 Acipenser. While I am inclined to doubt this derivation of the bone, 

 thinking it much more probable that it is derived from a scale-like 

 bone related to a lateral canal, it is evident that if it has, primarily, 

 the origin that Gegenbaur ascribes to it, it must contain, in Amia 

 and Polyodon, two distinct components, a membranous and a lateral 

 canal one ; provided, of course, that the sensory organs of the lateral 

 canals are actually centres of independent osteoblastic proliferation, as 

 Klaatsch (14) asserts. Whether the bone in Acipenser, also, contains 

 this lateral canal component I can not tell from the descriptions, but 

 in Menidia it would seem not to contain such a component, for Her- 

 rick (11, p, 37) says that the suprascapular of that fish is wholly 

 unrelated to the lateral canals. 



The short section of lateral canal enclosed in the supraclavicular 

 of Polyodon probably lodges a single sense organ of the main lateral 

 line, for there is a primary tube at each end of the section and no 

 intermediate one. In Amia the bone lodges two organs. 



Having left this bone, the canal, in Polydon, enters bone C^ of 

 Bridge's descriptions, this bone being considered both by that author 

 and by Collinge as the posttemporal, which bone is the suprascapular 

 of my nomenclature. The canal traverses this bone for a short di- 

 stance only, as shown in the accompanying figures, the short section 

 of canal enclosed in the bone probably containing a single sense 

 organ; for, as in the case of the supraclavicular, there is a primary 

 tube at each end of the section, and no intermediate one. 



Having left the canal in this bone the lateral canal continues 

 forward, as Collinge says, "in a series of small canal bones" ; but 

 these bones lie mainly dorsal to the anterior arm of C\ immediately 



