412 Proceedings of the Royal Physical Society. 



which have become too numerous to secure for themselves 

 an adequate nutritive supply ; this is not really an assump- 

 tion, for Waldeyer distinctly states that this is the mode of 

 formation in Placentalia; I can confirm this statement by 

 some observations of my own upon the ovary of the Potto ; 

 in the ripe follicles of this animal, the liquor folliculi con- 

 tained numerous nuclei which seem to be identical with 

 the nuclei of the follicular cells, and were presumably set 

 free by the dissolution of the cell protoplasm. 



The follicle of Phascolarctos is intermediate hetween that of 

 Phalangista and that of Didelphys and the higher mammals 

 generally. It is like Phalangista in the central position of 

 the ovum ; it is like the higher mammals in the complete 

 conversion of the superfl.uous follicular cells into an inert 

 liquid. It is a very slight step from this point to the follicle 

 of the higher mammalia. 



Tliere is thus some evidence that the marsupials are inter- 

 mediate hetween the Monotremata and the higher mammals, 

 not only, as Caldwell has pointed out, in the structure and 

 development of the ovum, hut also in the structure of the follicle. 



XXXIV. Notes on Carhoniferoiis Selachii. By Dr K. H. 



Traquair, r.E.s., r.G.s. 



(Read 18th January 1888.) 



The Cladodontid^. 



The teeth known as Cladodus (type C. mirahilis, Ag.) have 

 a flattened, transversely elongated, sub-elliptical or reniform 

 base, the anterior margin being straighter than the posterior 

 and often slightly excavated in the middle. Anteriorly the 

 base is thick, and generally shows a groove separating the 

 truly basal from the coronal portion, while the posterior 

 margin is thin, owing to the downward and backward slope 

 of the upper surface. From this upper surface anteriorly 

 spring a number of cones or denticles, of which the medium 

 is the longest; it is flanked by lateral denticles, of which an 

 outer one on each side is longer than those intermediate. 

 The larger denticles, at least, are flattened antero-posteriorly, 

 .and -have lateral cutting margins. 



