br R. J. TILLYARD. 8^ 



j The three dark pigmeiit-Vjiuid.s barely indicated; lateral gills lOiiini. 

 '2. A loJifeS median 9 nnii A. c'tnyiilafux Burni. 



I The three dark pigment- bands verj-^ conspicuous 8. 



[Lateral gills 9 mm. long, median slightlj' less A. p.syche tSelys. 



'^- ^Lateral gills 7 mm. long, median slightly less A. hda tSelys. 



Ill addition, the New Zealand A colensoiiis White, lias the 

 gills vt!ry long and narrow, with pointed tips; the median gill is 

 longer and more pointed than in A. aaalis. 



In Plate i., I have figuied tlie lateral gill of A. psyche (fig.l) 

 and the median gill of A. cluyulatus (tig. 2), together with the 

 tip of the same gill of A. colensonis for comparison. 



(ii.) The Agrioaid Fomn of Vertical Lamellar Gill. 

 (Text-figs.22-26; Plate i., fig. 7; Plate ii., figs.12-20; Plate iii., 

 figs.26, 27: Plate v., figs.38, 39). 

 These gills exhibit immense variety in shape and appearance, 

 hut all show the same internal structure. All the Vertical 

 Lamellar Gills of Ayrioiii.d(P so far examined by me are either 

 two-jointed, or show undoubted signs of having been so origin- 

 ally. The principal characteristics which separate the Ayiionid 

 gills from the Lestid forms are as follows : — 



1. Both median and lateral gills have two mid-ribs, right and 

 left. 



2. Both median and lateral gills carry hvo main longitudinal 

 tracheae. But, whereas both these pass through the base of the 

 median gill, only one enters the base of the lateral gill, the 

 bifurcation occurring just inside the gill itself. 



3. Although the blood-canals lie dorsally and ventrally on 

 either side of the main tracheaj (as in the Lestidce), yet the rachis 

 is not so definitely constricted off into three portions as it is in 

 the Lestid gill. Large secondary internal laminae are sometimes 

 developed in the rachis, between the blood-canals and the main 

 tracheae. 



4. The principal internal laminae are well-developed. Numer- 

 ous secondary laminae are developed in both dorsal and ventral 

 portions of the gill-blade. 



5. In the median gill, all four principal nerves are present, in 



