323 



The prefrontal fontanelle of Gegenbaur's descriptions of the 

 Notidanidae and Spinacidae consists of two morphologically wholly- 

 different parts ; a perforation of the anterior cranial wall which is filled 

 with a jelly-like connective tissue (gallertartiges Bindegewebe), and 

 a large open depression which lies immediately anterior to that perfora- 

 tion, on the dorsal surface of the ethmoidal cartilage. The perforation, 

 alone, is the homologue of the entire fontanelle, or fenestra praecere- 

 brahs, of the Carchariidae and ScylHidae, while the depression is the 

 homologue of the cavum internasale of Gaupp's descriptions of amphi- 

 bians. 



As this prefontanelle depression, or cavity, in selachians, has a 

 floor, and as that floor lies wholly anterior to that portion of the basis 

 cranii that corresponds to the planum internasale of Gaupp's des- 

 criptions of amphibians, the term cavum internasale, for the cavity in 

 selachians, seems inappropriate if the term planum internasale is to 

 be retained as at present used. The terms cavum and planum meseth- 

 moidale would definitely define the position of the cavity and its floor, 

 respectively, in fishes, but as the cavity Hes directly anterior to the 

 cavum cranii and even, in the prepared selachian skull, often appears 

 as a direct anterior extension of that cavity, cavum praecerebrale and 

 solum cavi praecerebralis seem much more appropriate terms. The cavity 

 has, moreover, already been referred to by Parker (1876), in Eaia, as 

 the "praecranial or internasal vacuity," and Hoffmann (1912) calls 

 it the " Präcranialhöhle" in his descriptions of the Pristidae and Pristio- 

 phoridae. The term solum instead of planum is proposed with the 

 intention of Hmiting the part of the basis cranii so designated to the 

 floor of the cavum praecerebrale, and also because Gaupp employs 

 the term planum praecerebrale (Praecerebralplatte) to designate that 

 portion of the anterior cranial wall that is formed, in certain amphi- 

 bians, by the chondrification of the tissues that fill the fenestra 

 praecerebraHs. This praecerebral plate of amphibians, it is to be 

 noted, is a vertical plate that hes definitely between the fenestrae ol- 

 factoriae, the planum internasale being a horizontal plate that also 

 Hes definitely between those same fenestrae and not necessarily 

 between the nasal capsules, and both plates might be designated by 

 terms that would express this relation to the fenestrae. 



The septum nasi of those selachians in which it is found lies 

 definitely ventral to the united trabeculae, and accordingly can not 

 be the homologue of the septum nasi of ganoids, teleosts and 



21* 



