Dr. Traquair — On Psejohoclus magnus,from E. Kilbride. 339 



In this work ' Mr. Davis follows IM'Coj'' in assigning three principal 

 teeth to each ramus of the jaw of Psephodus magnus, and has given 

 a figure representing three such teeth placed together in the position 

 he supposed them to occupy. Admitting Helodus planus as a 

 synonym, he assigns to these " Helodont " teeth a position — some in 

 front of the larger plates, others on the palate between them. 



Mr. Davis summarizes his views as follows : — " That a row of 

 three principal teeth, increasing in size backwards, were attached to 

 each cartilaginous ramus of the jaw ; that the diameter of the jaw, 

 as indicated by the groove or channel on the under surface of the 

 teeth, diminished towards the symphysis ; that a long narrow tooth 

 was placed in front of the anterior one, and that a series of at least 

 three Helodoid teeth were placed behind it, extending over the palate 

 and increasing in size backwards." (op. cit. p, 47.) The follewing 

 sentences may also be quoted : — " It is not probable that more than 

 three teeth existed on each side of the jaw ; and the teeth hitherto 

 known as Helodus planus, Ag., occupied the intervening portion of 

 the palate. An example (pi. Iv. fig. 6) shows a Helodoid tooth to 

 have occupied a position in front of the anterior tooth, as well as 

 the palatal space behind." (op. cit. p. 4:41.) 



These views appear, however, to be in the main hypothetical. 

 The " three principal teeth " have not, so far as I am aware, been 

 found together in the positions ascribed to them ; nor does it seem 

 clear what Mr. Davis means by the " intervening portion of the 

 palate," or the " palatal space," unless he supposes these Helodont 

 teeth to have been unsupported by the cartilaginous jaw arches, 

 mandibular below and palato-quadrate above, which is certainly im- 

 probable. One of his figures (pi. Iv. fig. 4), however, shows three 

 of the Helodoid teeth in longitudinal apposition; another (fig. 9) 

 two similarly related to each other, and to a bi'oad tooth ; and again 

 (fig. 6) we have one of the narrow teeth lying alongside the trigonal 

 one, supposed by M'Coy and himself to be " anterior." 



Mr. Davis's paper was read before the Koyal Dublin Society, 

 May 15, 1882, and published during the following year. But also 

 in the same year (1883) appeared the seventh volume of the Geolo- 

 gical Survey of Illinois, containing extensive new researches among 

 the Carboniferous selachian remains of that region by Messrs. 

 St. John and Worthen, and made independently of Mr. Davis's 

 work. In this volume a considerable share of attention is given to 

 the genus Psephodus, and the results certainly do not correspond 

 with the views of Mr. Davis. While the authors state that 

 " in reference to the dentition of Psephodus our information is 

 still meagre," they seem to have come to the conclusion that " while 

 the median portion of the rami of the jaws of Psephodus was en- 

 veloped by a moderately-contorted dental plate, this plate was 

 flanked on either side by series of teeth disposed in rows from within 

 outwards, similar to the occurrence of the teeth upon the jaws of 

 Cestracion." And as regards the allocation of the larger dental plates, 



' " On the Fossil Fishes of the Carboniferous Limestone Series," Sc. Trans. 

 Eoy. Dub. Soc. vol. i. ser. 2, 1883. (Eead May 15, 1882.) 



