IX] THE RESPIRATORY SYSTEM 185 



The lamellae in a hemibranch overlap from before backwards 

 (cf. fig. 79 B, E), and can be raised or lowered. They are prevented 

 from damaging one another by the presence of three lamellar 

 tubercles (fig. 82), one on the posterior surface (tp) towards the 

 tip, and two on the anterior surface (to) near the middle and base 

 respectively. Fig. 82 shews three consecutive gills of Austrocor- 

 dulia refracta, viewed in profile. 



The secondary modifications in type may be briefly noted and 

 named : 



(i) Archi-lamellate sub-type. This occurs only in the Synthe- 

 mini (fig. 79 E). There are only twelve widely-separated lamellae 

 in a hemibranch, and the basal pads are of enormous size. 



Fig. 82. Three lamellae from Austrocordulia refracta Tillyard ( x 87) seen in 

 profile, b base; bp basal pad; lam blade of lamella; svp supra-vortical 

 papilla; ta anterior tubercles; tp posterior tubercle. Original. 



(ii) Neo-lamellate sub-type (fig. 79 F). . Here the number of 

 gills is greatly increased ; they are placed close together, and the 

 basal pads are much smaller. 



(a) Corduline form. The lamella is irregular in outline, 

 somewhat hump-backed or nodding at the tip (fig. 81). Each 

 gill only overlaps the one following it. A very lovely mauve or 

 pale purple pigment is nearly always present. 



(6) Libelluline form. The lamella becomes nearly regular in 

 form, with broadly rounded tip (fig. 79 F). Each one overlaps 

 two or more following it. The tracheal vortex is weak or nearly 

 absent. The gills may be pure white (Orthetrum,), or pigmented as 

 in (a), or even of a deep shining black colour (Diplacodes). 



In Austrocordulia (fig. 80, 9) there is an extraordinary develop- 

 ment of a strong short terminal or supra-vortical papilla, armed with 

 curved spines (fig. 80, 10 ; fig. 82, svp). It is clearly an evagination 

 comparable with the papillae of Anax. 



Main-folds are completely absent in the Lamellate Type. 



