88 



CAUDAL GlLLS OF ZYGOPTEtllt) LMiVM, 



curved downwards towards tlie tip, and a little broadened at the 

 base. Three transverse zones of dense pigmentation are usually 

 to be distinguished, viz., one about one-fourth the length of the 

 gill from tlie base, another about half-way, and a third not far 

 from the tip. 



It is interesting to note that regenerated Lestine gills take on 

 a form closely resembling the gills of Synlestes, being much 

 shorter, pointed at the tips, and with the branch-tracheae running 

 obliquely out from the main stem. The number of main longi- 

 tudinal trachea? is usually doubled in the regenerated gill. This 

 would appear to be strong evidence in favour of the descent of 

 the Lestitie gill-form from the older Synlestine form, and of both 

 of these from the still older Triquetro-quadrate type, with double 

 the number of main tracheae. 



In life, the larvte of Lestinct hold their gills very wide apart, 

 the laterals at an angle 

 of from 45° to 60° to one 

 another, the median being 

 held high above them at 

 an angle of from 60° to 

 75°. As in all vertical 

 lamellar gills, the mid- 

 ribs do not divide the 

 gills into two equal por- 

 tions, but run asymmetri- 

 cally — in the lateral gills, 

 nearer to the ventral 

 edge; in the median gills, 

 nearer to the dorsal edge, 

 as shown in Text-fig. 21. 



The larvfe of four Australian species of Austrolestes are known 

 to me. Their gills can be distinguished as follows :— 



Lateral gills about 9 luiu. long when full-giown, with well-pointed 

 tips; much longer than median gill (6 "8 mm.), which is wider, 

 with rounded tip. The three dark pigment-bands only faintly 



indicated '• • 1- A- analis Ramb. 



Lateral gills with well-rounded tips, only slightly longer than 

 median '^' 



Text-tig. 21. 



ills of A(t.sti'ohHte.'< cliif/ii/af as {Bnvm.), in 



natural position, lateral vieM"; ( x^J..) 



