386 Mr Tims, Horny Teeth of the Marsipobranchn. 



pocket, causes the wall of the pocket of one scale to lie partially 

 subjacent to the superposed scale. If by a little crowding, such 

 as would be caused by the involution into the mouth, the super- 

 position became comj:>lete instead of partial, then there would be 

 precisely the same relative position of parts as is shown by Beard 

 to exist in Petromyzon. 



The above hypothesis seems to me to be also quite compatible 

 with the main facts, as given by Beard and Warren, of the struc- 

 ture and disposition of the teeth in Myxine and Bdellostoma. In 

 these animals the conditions have become somewhat more 

 specialised, the dental papilla better marked and its cells more 

 differentiated to form the so-called " odontoblast cone." If such 

 be the case, it furnishes further evidence in favour of the correct- 

 ness of Huxley's view that the Petromyzontidae are the most 

 primitive among the Cyclostomes. 



REFERENCES. 



1. Beard, J. "The nature of the teeth of the Marsipobraneh fishes." 



Morph. Jahr. ill. 1889, pp. 727—753. 



2. Howes, G. B. " On the affinities, inter-relationships and systematic 



position of the Marsipobranchii." Proc. and Trans. Liverpool Bio- 

 logical Soc. Vol. 6, 1892, pp. 122—148. 



3. Warren, E. "On the Teeth of Petromyzon and Myxine." Quart. 



Journ. Micr. Sci. Vol. 45, 1902, pp. 631—637. 



4. Tins, H. W. Marett. " The development, structure and morphology of 



the Scales in some Teleostean fish." Quart. Journ. Micr. Sci. Vol. 49, 

 1905, pp. 39—68. 



