Dr. Traquair — On PsejjJiodus mag nus, from E. Kilbride. 343 



in the recent Cestracion, which all cross the jaw obliquely from 

 within outwards and forwards — a direction precisely opposite to 

 what is seen in Mr. Davis's hypothetical figure of the dentition of 

 Psepliodus. Mr. Davis seems to be quite aware that, in placing the 

 teeth so, he has departed from the analogy of CocJiliodus, for, while 

 he admits that in that genus and in CochliodontidEe generally, the 

 inrolled teeth increase from the inner side, he states that in Psepliodus 

 the increase takes place along the opposite or outer side {op. cit. p. 

 416). It does not seem clear on what reasons this opinion is founded.^ 



The imaginary nature of Mr. Davis's restoration is further 

 illustrated by the fact that in the present specimen the two teeth 

 of the form supposed by Mr. Davis to be " posterior " and " middle " 

 are nearly exactly of the same length. This is indeed what we might 

 expect if they belonged to upper and lower jaws, whereas in the 

 figure referred to, the supposed " middle " tooth is, to suit the 

 tapering form of the jaw, much smaller than the " posterior " one 

 behind it. 



On the whole, I consider that view to be most likely to be correct, 

 which would ascribe to the mouth of Psepliodus four large tooth- 

 plates — two above and two below — each occupying on the ramus a 

 position similar to that of the row of largest teeth in Cestracion, or of 

 the so-called median tooth of Cochliodus, but that of the upper jaw 

 dilBfering slightly in form from the one opposed to it below. If Messrs. 

 St. John and Worthen's allocation of these larger teeth be correct, 

 the tooth (a) of the present specimen will belong to the right half of 

 the middle jaw, while (6) will be that of the left half or ramus of 

 the mandible. 



The subtrigonal tooth considered by M'Coy and Davis as 

 "anterior " does not occur in our specimen, but it clearly appertains 

 also to Psepliodus, and there is no reason why it should not also have 

 occupied a position in front of the large ones (a or &). 



Teeth of different forms seem to have been present in the jaw 

 external to, and in front of the large plate ; those in its immediate 

 vicinity belonging more or less to the category of Selodus planus, 

 including the peculiar forms figured (e),and that resembling ifeZocZtts 

 rudis. But the front of the jaw was armed with small teeth belong- 

 ing to the type, which has been designated as Zophodus by Eoman- 

 owski, and the forms didymus and Icevissimus, both named as species 

 of Helodiis by Agassiz. Hence, not only must those species be 

 merged in Psephodvs, but the genus Lophodus, founded upon H. 

 didymus and allied forms, ceases to have any but a conventional 

 existence. It would, I think, have been well, before separating ofi" 

 any new genera from the old Helodus of Agassiz, to have sought 

 for more precise information as to the essential characters of the 

 Coal-measure species H. simplex, which, as it occurs first both in 



1 In placing the long or obtusely angulated margin of the supposed "posterior" 

 tooth {b of our specimen) outwards, Mr. Davis apparently follows M 'Coy's description. 

 But in describing the supposed "middle" tooth («) M'Coy calls its long margin 

 "inner," and though Mr. Davis quotes this description verbatim, he, nevertheless, 

 with some inconsistency places the long margin in exactly the reverse position, 

 namely, external. 



