EASTMAN : STRUCTURE AND RELATIONS OF MYLOSTOMA. 15 



nearer approach to Ctenodipterine conditions has been noted by Cope * 

 in Macropetalichthys, where the space occupied by the chondrocraninra 

 is closed posteriorly by a thin osseous septum extending transversely 

 across the headshield, and pierced in the middle by a triangular opening 

 for passage of the notochord. The same genus is further remarkable 

 for having a partially ossified parasphenoid, which is produced posteriorly, 

 beyond the transverse septum referred to as far as the hinder margin of 

 the headshield, and supports the forward portion of the vertebral axis in 

 a manner analogous to that in Neoceratodus and sturgeons. Mention 

 may also be made in this connection of an undescribed Coccostean from 

 the Devonian of Wildungen, which, according to Jaekel, 2 shows traces of 

 an " echte Schadelkapsel mit deutlichem Occiput und Foramen magnum." 

 Aside from these instances, no ossifications are known within the interior 

 of the headshield of Arthrodires which can be associated with the 

 chondrocranium. The presence of a pineal gland is distinctly indicated 

 in most forms, its position being as in Neoceratodus. 



Another point of resemblance between Arthrodires and typical 

 Dipnoans is found in the disposition of the external nares. Dipnoans 

 represent an advance over Elasmobranchs in that the oro-nasal grooves 

 of the latter are converted into true nasal passages, and the olfactory 

 pits communicate with the mouth by internal nostrils as in higher 

 vertebrates. This modification is evidently to be associated with air- 

 breathing habits, and, as pointed out by Huxley 8 , suggests that the 

 sense of smell is of value to these creatures. The occurrence of internal 

 narial openings amongst Arthrodires would coincide with other evidence 

 indicating Dipnoan relationships, and we may even attempt to define 

 their position in at least one genus, as will be noted in the description 

 of Dinomylostoma (p. 27). The large size of the supposed olfactory 

 capsules in Dinichthys 4 may also be considered as implying a tolerably 

 keen perception of odoriferous particles, which is dependent upon the 



1 Cope, E. D. On the characters of some Palaeozoic fishes. Proc. U. S. Nat. 

 Museum, 1891, 14, p. 453, Plate 29, Fig. 4, Plate 30, Fig. 5. 



2 Jaekel, O. Ueber die Organisation und systematische Stellung der As- 

 terolepiden. Zeitschr. Deutsch. Geol. Gesell., 1903, 55, p. 48. 



3 Huxley, T. H. On Ceratodus forsteri, etc. Proc. Zool. Soc. London, 1876, 

 p. 27. Position of the anterior nasal apertures in Lepidosiren. Ibid., p. 180. 



4 These are described as " optic capsules " by Newberry in his Monograph 

 on the Palaeozoic fishes of North America, p. 146, Plate 7, Figs. 2, 2a. They are 

 stated to occur "not always in the same position, but they were two in number, 

 one on each side, and located well within and near the anterior extremity of the 

 head." 



