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bulletin: museum of comparative zoology. 



coordinated with those of typical Coccosteans, allowance being made for 

 the fact that the preorbital in the modern form appears to have 

 reverted to its primitive cartilaginous condition. Similar instances of 

 reversion, or degeneration of membrane-bones, occur among Chondros- 

 teans and Teleosts. A tendency toward reversion would seem to be 

 indicated also by the external occipital along its inner border, and the 

 small plates surrounding the orbits show imperfect ossification. The 

 latter, according to the observations of Huxley, Traquair, and others, 



are sometimes incon- 

 stant ill number, the 

 suborbitals tending to 

 become fused into a 

 single piece not unlike 

 that of typical Cocco- 

 steans {cf. Figs. C, D). 

 The postorbitals of 

 Neoceratodus are in- 

 terpreted by Bridge ^ 

 as remnants of an 

 obsolescent supraor- 



FiG. C. — Left suborbital plate of Blmchthys terrelli 

 Newb., from the Cleveland shale of Ohio. External as- 

 pect, X ^. 



Fig. £>. — The three plates forming 

 collectively the suborbital of Neocerat- 

 odus Jorsteri (Krefft), drawn from the 

 same specimen as shown in Fig. A. x i. 



bital ring ; by Fiirbringer ^ as bones 

 that have become newly formed about 

 the sensory canals bounding the orbit, 

 yet belonging properly to the cranial 

 roof. The explanation here offered 

 is that they are equivalent to the 

 single large postorbital occurring on 

 either side of the headshield in 

 Arthrodires. 



Unlike Ctenodipterines, Neocera- 

 todus retains throughout life a completely closed and almost entirely 

 unossified chondrocranium, and this notable peculiarity would seem to 

 be shared also by most Arthrodires. In Chelyophorus, however, two 

 small ossifications occur in a position corresponding to the exoccipitals, 

 and have been interpreted as such by Smith Woodward.^ A somewhat 



1 Bridge, T. W. Morphology of the skull in Lepidosiren, etc. Trans. Zool. 

 Soc. London, 1898, 14, p. 355. 



2 Fiirbringer, K. Beitrage zur Morphologie des Skeletes der Dipnoer. Jena 

 Denkschr., 1904, 4, p. 445. 



3 Woodward, A. S. Catalogue fossil fishes British Museum, 1891, pt. 2, p. 280. 



