270 



In Selachians there are, as is well known, no maxillary bones, and not 

 even any maxillary teeth (No. 34), but the maxillary labial is con- 

 sidered by Gegenbaur (No. 15, p. 223) as the underlying cartilage 

 in relation to which the maxillary bone of Teleosts and higher animals 

 is developed, the so-called maxillary bones of all vertebrates being 

 considered by him as homologous structures. If, however, these bones 

 are not all homologous structures it is evident that it must be the 

 Teleostean maxillary and not the maxillary of higher vertebrates, that 

 is the uue that is developed in relation to the selachian maxillary 

 cartilage. The labial of Polypterus may, accordingly, be the homologue 

 a part of one or the other of these two labial cartilages of Selachians. 



The maxillary and premandibular labials of Selachians are con- 

 sidered by Gegenbaue as parts of a premandibular visceral arch. 

 The folds in which they lie being quite probably the homologues of 

 the maxillary and mandibular folds of Teleosts, and hence also of 

 Ceratodus, the opening leading from the supramaxillary pocket, or 

 labial cavity (Günther), of the latter fish into the mouth cavity, said 

 by Marco to have been found in one of his specimens, must be a 

 representative of a premandibular and not of a postmandibular cleft. 

 That this pocket, and the supramaxillary furrow of Teleosts, can be 

 the homologues of the lachrymal groove of higher vertebrates seems 

 to me most improbable; and this, if true, is evidence against the 

 assumption of the homology of the parts separated by the pocket or 

 furrow with the maxillary and lateral nasal processes of higher 

 animals. 



Turning now to Teleosts, the maxillary bone is found to be not 

 only very variable in itself, but also to vary greatly in its relations 

 to the premaxillary. According to Sagemehl (No. 37, p. 101) the 

 maxillary was primarily a tooth-bearing bone that lay "lateral" to the 

 premaxillary, and formed the lateral portion of the upper margin of 

 the mouth. By this it is evidently meant that the maxillary bone lay, 

 primarily, postero-lateral to the lateral end of the premaxillary, and 

 formed a direct continuation of the line of that bone along the upper 

 edge of the mouth. Secondarily the bone acquired, according to Sage- 

 mehl, the position "behind", or, more properly, internal to the pre- 

 maxillary, this position being said to be the one found in by far the 

 largest number of Teleosts. When the bone is found in this position 

 it is said to be, without exception, toothless. The position of the 

 maxillary bone here defined by Sagemehl as lateral is typically re- 

 presented in Amia and Erythrinus, in both of which fishes, however, 

 the anterior articular end of the maxillary turns inward behind and 



