550 



PROP. E. W. CLAYPOLE ON THE STETTCTUEE 



doubt can be entertained regarding the original position of the 

 two shells above described. They are represented in the accompany- 

 ing figures. 



Fig. 1. — First specimen, showing both shields in exact juxtaposition. 



Pig. 2. — Second specimen, 

 showing both shields in 

 juxtaposition. 



[Section in front of laterals.] 



As will be seen from a careful examination of the figures, there 

 is not here even the slight lateral slip which has, to a small extent, 

 displaced the plates in Yon Alth's Pterasjpis. The two shields fit 

 accurately and closely one upon the 

 other. Nor do they show any trace 

 of the flange or outspread margin 

 which is represented in the section 

 by Yon Alth ; but the edges, which 

 in the dorsal plate at least are some- 

 what thickened, fit exactly on to each 

 other as a lid on a box. So close an 

 accuracy of fit may be quoted as a 

 further argument in favour of the 

 belief that tbis was their position 

 during the life of the fish, as it is 

 exceedingly improbable that after 

 being once shifted they would ever 

 come again into juxtaposition so precise. 



It then follows that Palceaspis bitruncata ceases to exist as a 

 specific individuality, and becomes, as Scaphaspis has already 

 become, only the name of the ventral plate of Palceaspis americana.^ 

 The absence of any notch for the eye, previously attributed to an 

 imperfection of the specimen, and the usually rather smaller size 

 are thus at once explicable. 



When I described Palceaspis in 1884 I was unaware of the 



[Place of section nncertain.] 



^ Since the above was written, T have received Mr. A. Smith Woodward' 

 ' Catalogue of the Fossil Fishes in the British Museum,' and notice that he has 

 made a suggestion to the same effect, and has in consequence treated one of 

 these fossils as the ventral armour of the other. His foresight thus proves 

 entirely just. 



