8 bulletin: museum of comparative zoology. 



able simplification is to be observed amongst modern Sirenoids, in that 

 the articular element is not differentiated from the Meckelian cartilage, 

 the ensheathing angular has become reduced in the two more specialized 

 genera to a mere splint-like rudiment, and in the same genera the flat 

 triangular piece called by Huxley the "dentary," by Furbringer 1 the 

 "submandibular," has disappeared entirely. Still further reduction is 

 evident amongst Arthrodires, where there are no bones ensheathing 

 Meckel's cartilage externally, and the only ossifications thus far recog- 

 nized consist of the splenial and mandibular dental plate. All the best 

 known genera display a conspicuous groove along the anteroinferior 

 border of the splenial, passing underneath and to the inner side of the 

 dental plate proper, and terminating a little short of the symphysis. 

 Its general appearance, position, and direction at once recall the very 

 similar groove in Protopterus, hence it is natural to attribute to it a 

 corresponding function. In it were lodged remnants of the Meckelian 

 cartilage, precisely as in living forms. 2 



The suggestion has been made by one or two recent writers that the 

 jaws of Arthrodires are non-homologous with those of ordinary fishes. 

 Dean, for instance, supposes them to have originated from merely dermal 

 ossifications, and to be in nowise derived from visceral arches. Unessen- 

 tial structural differences, and assumed functional differences, such as 

 mobility of the mandibular rami in a manner wholly unique amongst 

 Chordates, are urged in support of this novel idea. Whether we com- 

 pare the Arthrodiran lower jaw with that of Ctenodipterines, as 

 Sagenodus, 3 for example, or of modern Sirenoids, the obvious similarity 

 of all the parts, relations of the Meckelian cartilage, and insensible 

 transition between the splenial and mandibular dental plate as regards 



1 Furbringer, K. Beitrage zur MorphoSogie des Skeletes der Dipnoer, etc. 

 Semon's Zool. Forschungsreisen in Australien. Jena Denkschr., 1904, 4, p. 442. 

 Miall and Traquair employ Huxley's designation ; the same element is also named 

 " predentale " by Van Wije, and " dermomentale " by Fritsch. Its origin appears 

 to be conditioned by the presence of mandibular sensory canals, the bone being 

 formed around them. When canals are lacking, as in Protopterus and Arthrodires, 

 no submandibular occurs. 



2 Furbringer, K. Op. cit., p. 481, Plate 39, Fig. 28. Wiedersheim, R., Morpholog. 

 Studien. Jena, 1880, 1, p. 55, Plate 2, Figs. 3, 8. 



8 The splenial, for instance, is notably elongated in the form figured by T. Atthey 

 under the name of Ctenodus obliquus in the Ann. Mag. Nat. Hist., 1875, ser. 4, 

 15, p. 390, Plate 19, Fig. 2. By Smith Woodward this species is considered iden- 

 tical with Sagenodus inaequalis Owen. In Sagenodus pertenuis, from the Permian of 

 America and Russia, the dental plates develop sharp cutting edges. See Amer. 

 Nat., 1904, 37, p. 493-495. 



