Devonian Fi'sh-P^tnna of f^pttzhergen. 15 



Eifcl. There is a large keelcil plate of a very similar charac- 

 ter from the neighbourhood of Gcrolstein in the J^Iuseuni of 

 Comparative Zoology, Cambridge (Mass.) ; and evidence is 

 gradually accumulating to ]n-ovo that certain of the Ostraco- 

 dcrmi attained comparatively gigantic proportions. 



EXPLANATION OF THE PLATES. 

 Plate I. 



Fii/.l. Acanthaspis dccipiensj sp. n. ; associated dorsal (a) and vcntinl 

 (b) shields. Lower Devonian, Dickson I3ay. a.v.L, anterior 

 ventro-lateral ; 6, basal ])late of pectoral appendajic : c, inipros- 

 sion of ridge and constriction on tli(> visceral aspect of the pos- 

 terior ventro-laterals; /;/, longitudinal lateral ridge (and supposed 

 suture) on the dorsal sliield ; m.v., median ventral ; p.v.L, ])os- 

 lerior ventro-lateral ; .";, pectoral appendage ; x, pair of descend- 

 ing processes on visceral aspect of dorsal shield ; ;/ ~, line ui 

 transverse section, fig. 2. 



Fiy. 2. Ditto ; transverse section of dorsal shield of the same specimen, 

 along // ;:. 



Fiy. 3. Ditto ; two transverse sections of the pectoral appendage of the 

 same specimen, one («) nearer the base than the other (b). 



Plate II. 



Fig. 1. Pteraspis Nathorsti, Lank. ; hinder portion of ventral shield, 



visceral aspect. Lower Devonian, Dickson Bay. 

 Fiff. 2. Acantliasjn's minor, sp. n. ; portion of occipital plate, visceral 



aspect, twice nat. size. Lower Devonian, Dickson Bay. /», 



median process of liinder border. 

 Ficf. 3. Ditto ; portion of pectoral appendage, associated Avith above, 



twice nat. size. 

 Figs. 4, 5. Ditto ; two plates of the same, twice nat. size, outer and inner 



aspect respectively. 

 Figs. 6-10. Poroh'pis posnanieyisis (Kade) ; scales. Lower Devonian, 



Klaas Billen Bay. Fig. G is inner aspect, nat. size ; the others 



exhibit the outer aspect, twice or tlirice nat. size. 

 Fig. 11. Psa))uno!iteus are?ia(us, Ag. ; portion of bent plate, front view 



and transverse section (a). Fig. 11 />. Abraded tubercles of 



flat plate of ditto, enlarged. Upper Devonian, ^timers Valley. 

 Fig. 12. Onychodus arciicus, A. iS. Woodw. ; presymphysial bone, twice 



nat. size. Upper Devonian, Mimers Valley. 



Plate III. 



Fig. 1. Astei-oplax scabra, g. et sp. n. ; cranial shield. Upper Devonian, 

 Mimers Valley, o, o,, median plates ; 1, 2, posterior and ante- 

 rior paired plates ; x, facial plate ; y, z, undetermined plates. 



Fig. 2. Ditto ; problematical bone exhibited on the inferior aspect of the 

 same specimen, two thirds nat. size. 



The specimens are all preserved in the Royal Swedish State Museum, 

 Stockholm, and unless otherwise stated the figures are of the natural size. 



