BY R. J. TlLLYAkD. 87 



closer together. Their nuclei do not exceed 8-9// in length. 

 Pigmentation of the hypodermis is usually only moderately dense, 

 but, in some forms, dense bands of black pigment run across the 

 gill at definite levels. 



The Lestid gill-form is divisible into two very distinct sub- 

 groups, each representative of a single sub-family. 



In the Syidestinoi (genus Synlestes, Text-fig. 20; Plate i , figs. 

 4-5; Plate iii., fig. 25), the gills are short, or only of moderate 

 length. The branch-tracheae come off from the main longitudinal 

 stems obliquely. Bands of dense black pigment are absent. 



Synlestes iveyersi Selys 



(Text-fig.20; Plate i..fig.4) 



has the gills about 6 mm. 



long when full-grown, 



distinctly pointed at tips, 



and carrying a few long, 



slender hairs distally. "". 



rni T .11 . T 1 ,^ Gills of SynJestes n'eyersi Selys, in iiatuial 



The median gill is sbghtly •^. , ^ , • , n^ 



'^ o J position, lateral view; ( x b). 



shorter than the laterals. 



The three gills are held fairly close together, the angle between 



the two laterals being about 20°, and the median gill being 



raised only about 15° above them. 



Synlestes albicanAa Tillyard (Plate i., fig. 5) has the lateral 

 gills barely 4 mm. long when full-grown, broad and evenly 

 rounded at the tips, and without any long hairs. 'J'he median 

 gill is 0-5 mm. shorter, closely similar in shape to the laterals, 

 but somewhat more oval. The gills are held fairly close together 

 in life, as in S. iveyseri. 



In the Lestince (genus Austrolestes, Text-fig. 21; Plate i., figs. 

 1-3) the gills are excessively elongated, and usually with well- 

 rounded tips. No long hairs are present, but there may be a few 

 short ones near the tip. The branch-tracheae come off from the 

 main longitudinal stems practically at right a'iiyles, a character 

 peculiar to this subfamily. Transverse bands of dense black 

 pigment are frequently developed on these gills. 'J'he lateral 

 gills are usually quite straiglit, and of nearly the same width 

 throughout; the median gill is usually slightly shorter, a little 



