THE 



aEOLOGIGAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. IX. 



No. VII.— JULY, 1902. 



I. — Additional Note on Dbepanaspis Gemundenensis, Schluter. 

 By Dr. R. H. Traquair, F.R.S., F.G.S. 



IN my "Notes on Drepanaspis Gemundenensis, Schliiter," published 

 in the Geological Magazine for April, 1900 (pp. 153-159), 

 I described a paired plate of a trapezoidal form occupying a position 

 on each margin of the under surface of the head, a little behind the 

 mouth, and to which I gave the name ' sensory ' plate, as it is 

 perforated by an opening which, if not orbital, evidently marks 

 the place of an organ of sense of some sort. With this opening 

 I also identified a depression seen in some specimens in a similar 

 position on the dorsal aspect, and came to the conclusion that its 

 position was marginal, being seen in some specimens on the ventral 

 and in others on the dorsal surface of the head. I was also much 

 puzzled to find that in some specimens this pit or depression seemed 

 to have a floor, covered with tubercles like those of the rest of the 

 surface, a condition which apparently militated very decisively 

 against the idea of its being an orbit. 



Increase of material has enabled me to clear up this point at last, 

 and to rectify my restored figures accordingly. 



I found, namely, that when the appearance in question is seen 

 on the ventral aspect (x, Fig. 2), it is, in all cases, an actual 

 perforation, but when seen on the dorsal it is a mere depression 

 surrounded by a slightly elevated margin and provided with an 

 imperforate tuberculated floor. Isolated specimens of the sensory 

 plate show also that it was not reflected round the lateral margin 

 of the head (it is not so at least in the fossils), and when I obtained 

 a view of the plate from its internal surface, the perfoi'ation was 

 found to be surrounded by a thickened ring-like margin. Here, 



DECADE IT. VOL. IX,— 'KO. VII. 19 



