Mr Tims, Horny Teeth of the Marsipobranchii. 383 



A suggestion as to the nature of the Horny Teeth of the 

 Marsipobranchii. By H. W. Marett Tims, M.A. [Camb.], M.D. 

 [Edin.], King's College, Cambridge. 



[Received 30 April 1906.] 



The morphology of the teeth of the Marsipobranchii has more 

 than a passing interest, since it is intimately bound up with the 

 question of the phylogeny of the group. 



The view that the Cyclos tomes are degenerate gnathostomatous 

 vertebrates, degenerate as the result of parasitism, is supported 

 by all the authority of Huxley, Dohrn, Howes, Beard and others. 

 The opposite conclusion, that they are primitive vertebrates in 

 which true jaws have not been developed, has been advocated by 

 Balfour, Gegenbaur, Haeckel, and more recently by Ayres. 

 Further, while Huxley regarded the Petromyzontidae as the most 

 primitive of the Cyclostomes, Parker, Beard and others would 

 place the Hags in that position. 



Howes [2] has pointed out that whether or not the Cyclostomes 

 are to be looked upon as gnathostomatous depends upon the in- 

 terpretation placed upon the term, since it is used indifferently 

 to " express either the mere possession of jaws, apart from any 

 consideration of teeth or [more generally] that of a dentigerous 

 apparatus." 



The disposition and gross anatomy of the teeth have been 

 described by J. Mtiller, W. K. Parker and others, but Beard was 

 the first to investigate their histological characters [1]. Since 

 the publication of his paper others have appeared by Behrend, 

 Ayres and Warren, which, while agreeing in the main with 

 Beard's results, differ in several important details. 



While working recently at the morphology and development 

 of the scales of the Teleostei my attention was directed to the 

 subject of the teeth of the Cyclostomes, and it appears to me that 

 the results of those investigations throw some light upon the 

 nature and evolution of the cyclostome teeth, and to some extent 

 harmonise the apparent discrepancies. 



If one examines the scales of one of the Gadidae they arc 

 found to consist of a number of calcified scalclets arranged excen- 

 trically around a somewhat thickened and calcified "centrum" or 

 " foyer," the surface of which is smooth. All these calcified 

 structures lie upon a fibrous basis, the periphery of which is 

 turned upwards to become continuous with the scale-pocket or 

 "schuppen bache." The scale increases in size at its margin, that 



