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an enclosed maxillary bone, I was led to examine a most imperfect 

 specimen of Ceratodus, which I happen to have, to see if the same 

 conditions did not perhaps exist in that fish also. There is, in 

 Ceratodus, a most evident mandibular labial fold strongly and nor- 

 mally developed, and already described by other authors. The maxil- 

 lary fold is not so evident, and has not been described, but it never- 

 theless probably exists, and has simply been turned inward on to what 

 was considered by Günther (No. 16) as a part of the dorsal surface 

 of the mouth cavity. Huxley (No. 21), in 1876, showed that Günther 

 was wrong in considering the anterior and lateral portions of this sur- 

 face as a part of the mouth cavity. These parts of the surface lie, 

 in fact, outside the mouth cavity, form an under, concave portion of 

 the anterior and lateral parts of the head, and it is on this surface 

 that the maxillary folds lie. 



Each of these maxillary folds thus lies in a somewhat horizontal 

 position, its lateral edge corresponding to the free dorsal edge of the 

 fold in Polypterus ; and as the furrow that separates the fold from the 

 adjoining parts of the head has been much more developed in its deeper 

 parts than near the surface, it has thus given rise to a deep pocket. 

 This pocket has been described by both Günther (No. 16) and Margo 

 (No. 25), and the latter author is said, in an abstract of his work, 

 which is all I have been able to consult, to have considered it as the 

 homologue of the spiracle of other fishes. That it is, on the con- 

 trary, the homologue of the supramaxillary furrow of Amia seems to 

 me most evident, and reference to my figures of young larvae of Amia 

 (No. 1, Fig. 4) will show that the furrow in that fish, when it first 

 appears, is a sort of pocket near the hind end of the maxilla. 



There is thus quite probably, in Ceratodus, as in Polypterus, a 

 maxillary fold without either an associated maxillary bone or any 

 maxillary teeth. No indication whatever of this fold, or of the asso- 

 ciated pocket, is given by Semon (No. 39), either in his descriptions 

 or in his figures, and this would seem to indicate that it is of re- 

 latively late formation, as it is also in Amia, not appearing until the 

 larvae are over 10 mm in length. 



In Selachians there are, as is well known, what are called by 

 Gegenbaur (No. 15, p. 212) "Mundwinkelfalten". These folds some- 

 what resemble, in general outward appearance, the maxillary and man- 

 dibular labial folds of Polypterus, and are doubtless their homologues. 

 At the point where the two folds unite, the maxillary and premandi- 

 bular labial cartilages of Gegenbaur's descriptions usually articulate 

 with each other, or are connected by ligament without articulation. 



