564 H. H. SWINNBRTON. 



unossified crania if their adult forms aloue were examined 

 The study of the development of the ossified vertebrate skull, 

 however, eliminates the difficulty, and satisfactorily proves that 

 the adult crania of the lower Vertebrata are but special develop- 

 ments of conditions through which the embryonic crania of 

 the highest members of the sub-kingdom pass " (p. 420). 



Given a brain requiring protection, it is quite as reasonable 

 to suppose that specialisation for this purpose would, in one 

 case, show itself by an increase of cartilage, as in another by 

 the accession of boue ; and that the presence of bone would 

 tend to preserve features belonging to the primitive cartila- 

 ginous cranium, which otherwise would be obliterated by 

 excessive growth processes. But suppositions and assumptions 

 must not displace facts. 



Comparing the adult cranium of Amia with that of 

 Heptanchus, it is seen that in the latter, except for nerve 

 foramina, the ear cavity is completely shut off from the 

 cavum cranii; and that the only fontanelle present is the 

 " Praefontalliicke," which lies at the anterior end of the 

 roof, and, according to Sagemehl, in front of the epiphysis. 

 In the former the cranial roof is massively cartilaginous, 

 continuous, and unfenestrated ; in the orbital region there is 

 a '^ considerable vacuity " closed by membrane, and regarded 

 by Sagemehl as a much enlarged optic fenestra (84, p. 202) ; 

 and in the auditory region there is a gap, between the cavum 

 cranii and the ear-capsule, which owes its existence, according 

 to the same authority, to the enlargement of the labyrinth ; 

 and, finally, the floor is occupied largely by a well-developed 

 pituitary fontanelle. Thus it becomes evident that, notwith- 

 standing the presence of a bony covering, the one fontanelle 

 present in the Selachian skull is absent in Amia; and those 

 fontanelles which are present in this are represented, at the 

 most, only by small nerve foramina in that. 



In the advanced Amia larva (19 mm. long), as in the young 

 salmon (6g. 62) and Lepidosteus (Parker, 81), a large fon- 

 tanelle certainly is present in the roof, but it cannot be 

 homplogised with the '' Prasfontalliicke," for it lies entirely 



