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described in the present work: in Scopelus of the Scopelidae, Supino ('01/02) shows a large myodome, 

 and I find one in Saurus griseus of the same family: in Crenilabrus pavo, of the Labridae, and in 

 Trachurus trachurus, of the Carangidae, I find a myodome well and normally developed: in Hoplo- 

 stethus, of the Berycidae, Supino ('04) shows a normal myodome, and Starks ('04, p. 602) gives, as 

 one of the characteristics of the Berycoidea, „Myodome large in front, closed abruptly behind, or 

 open to the exterior posteriorly only through a pore": in Fistularia, of the Aulostomi, it is said by 

 Swinnerton not to be developed, but, as already stated, this may be incorrect: in a Mediterranean 

 Sargus, and in Chrysophrys aurata, both of the Sparidae, I find a well developed myodome; as I also 

 do in a Mediterranean Xiphias of the Xiphiidae. 



It is thus seen that a myodome is found in certain living representatives of nearly all of the 

 earliest known f amilies of teleosts; and that certain of these f amilies, certain of the living represent- 

 atives of which possess a myodome, are found in earlier geological periods than any of the families 

 of the Teleostei the living representatives of which are known not to possess it. Certain of the Stylo- 

 dontidae, the earliest known representatives of the Lepidosteidae, are also said to possess a myodome. 

 The palaeontological record, as given by Zittel, thus certainly indicates that the conditions froin 

 which the myodome is developed are not to be looked for in either Lepidosteus or Silurus, but in 

 fishes belonging to earlier deposits than those in which those teleosts and ganoids that possess a myo- 

 dome are found; and the only fishes so found, living representatives of which are known, are, in 

 Zittel's terminology, the Selachii. Dipnoi, Chondrostei, and Crossopterygii. 



In the Selachii, Gegenbaur ('72) shows a thick interorbital wall, and a pituitary fossa (Sattel- 

 grube) which lies between, or but little posterior to, the posterior portions of the orbits. The canalis 

 transversus, which transmits the pituitary vein, lies in or beneath the bottom of this fossa (Hexanchus. 

 Mustelus, Galeus), or but slightly posterior to it (Heptanchus, Scymnus). In Ceratodus (Bing & Burck- 

 hardt, '05, P. 523) a similar interorbital wall and pituitary fossa are found; and the same is true of 

 Acipenser (Parker, '82a). In Polypterus there is a thick interorbital wall, and Pollard ('92) shows, 

 in the cranial cavity of this fish, a cartilaginous shelf which, as already stated, closely resembles the 

 proötic bridge of Amia. Bing & Burckhardt ('05, p. 571) show this bridge much more inclined than 

 Pollard shows it, and they show, as Pollard does, what is probably the Saccus vasculosus, projecting 

 backward beneath the bridge. No mention is made, in either Ceratodus, Acipenser or Polypterus, 

 of a canalis transversus or pituitary vein, but this vein must certainly exist in each of these fishes, 

 and must lead into the orbit of either side, as it does in the Selachii and Teleostei. Imagine the orbits, 

 in either one of these several fishes, to be enlarged and deepened. This would necessarily shorten 

 the canal traversed by the pituitary vein, and would, if sufficiently continued, bring the pituitary 

 fossa into the hind ends of the orbits, much as it is actually found in Dactylopterus and Gadus. Certain 

 of the eye-muscles would then almost necessarily have theirpoints of origin transferred to this pituitary 

 pocket, and a myodome would be established. 



This being accepted as the manner of origin of the myodome, do Lepidosteus and the Siluridae 

 present a primary or a secondary condition? It is evident that they might be considered as presenting 

 either one or the other, but it seems to me that both of them present a primary condition, for, as 

 already stated, if the anterior edge of the proötic bridge of Polypterus were to be bent downward 

 until it touched and coalesced, everywhere excepting in the middle line, with the underlying floor 

 of the cranial cavity, it would give rise to a condition closely resembling that found in Lepidosteus: 

 and if the cross-canal, thus produced, were to be invaded by the surrouncling cartilage until only 



