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a slight pit was left on its anterior surface, it would give rise to the conditions found in Ameiurus. 

 Under this assuniption the eondition of the myodomic region, as it exists, both in Lepidosteus and 

 Ameiurus, would be primary and not derived secondarily from a pre-existing myodome. But this 

 presupposes, if Ameiurus can be eonsidered as typical of the Siluridae, either that the Weberian 

 apparatus has been developed independently in the Siluridae and the other families of the Ostario- 

 physi, or that the myodome has been developed, in those families of the Ostariophysi in which it is 

 found, wholly independently of its development in other teleosts. For that a myodome could have 

 been developed from the eondition found in Ameiurus seems most improbable, Ameiurus quite cer- 

 tainly representing the end of a line in which the saccular portion of the myodomic region is aborting, 

 whether it be primarily or secondarily. 



Still another Suggestion regarding the myodome is that its basioccipital extension may have 

 been developed in relation to a vertebral depression on the anterior surface of that bone. In Trigla 

 lyra, I have shown that, that depression in the anterior end of the basioccipital that lodges the posterior 

 portion of the myodome is lined with a layer of dense bone that is similar to the b-vue that lines 

 the vertebral depression in the bind end of the same bone, and that these two linings of dense bone 

 are connected by a small median line of similar bone. This suggests, as I have already stated, that 

 the myodomic depression on the anterior end of the basioccipital of this fish is, like the depression 

 on its hind end, a vertebral depression, and if this be true of this fish, it must also be true of all other 

 fishes in which the myodome has a basioccipital extension. In Gadus and Saurus the myodome 

 has no basioccipital extension, but in both these fishes there is, nevertheless, a depression on the 

 anterior end of the basioccipital, and this depression — although lodging no part of the myodome — 

 would certainly seem to be the homologue of the myodomic depression of other fishes. This depression, 

 in Gadus and Saurus, is continuous with a large vacuity in the hind edge of the prootic, and if the 

 one is a vertebral depression on the anterior end of the basioccipital the Suggestion is evident that 

 the other might bea depression in someway related toa similar depression on the hind end of the proötic. 

 The proötic vacuity both of Gadus and of Saurus is in communication, by the intermediation of the 

 hypophysial fenestra, with the myodomic pocket on the anterior surface of the proötic. The sup- 

 position is thus evident that a myodomic pocket might have been first developed in relation to a 

 vertebral depression on the anterior surface of the proötic region, and that this depression, pushing 

 batkward and gradually obliterating or absorbing a similar depression on the posterior surface of the 

 same region, and then even occupying also a vertebral depression on the anterior end of the basioccip- 

 ital, has given rise to all known forms of the myodome. But this necessarily attributes a vertebral 

 origin to the basioccipital and proötic, for which, in the case of the proötic, there is no apparent 

 Warrant. Furthermore, according to Swinnerton's descriptions of the development of the basioccipital 

 in Gasterosteus, even the depression in the anterior end of that bone can not be a vertebral one. For 

 that author says ('02, p. 524) that, in that fish, a crest of membrane bone grows downward, in the 

 middle line, from the ventral surface of the primary portion of the basioccipital, and that ,,within 

 the substance of the fore part of this crest is a cavity which opens in front and receives the hinder 

 end of the externa! rectus muscle; this is the homologue of the anterior conical excavation of the 

 basioccipital of the Pike and many other teleosts". 



