BY R. J. Tir.LYARD. 



81 



curved towards the tip (Text-fig. 18). Now if a transverse sec- 

 tion be made anywhere along the gill, it will be seen to be 

 detinitely diamond-shaped, with the two upper sides of the sec- 

 tion equal and shorter that the two lower ones, which are also 

 equal. Thus it is quadrangular in section. The complete gill-sys- 

 tem, therefore, might be described as "triquetro quadrangular," a 

 cumbersome term which I have shortened to triquetro-quadrate . 



Owing to their long immersion in alcohol, I can 

 find little or no traces of the tracheal system in 

 whole mounts of my gills, though the collapsed 

 tracheje are easily seen in sections. As far as I am 

 able to make it out, the general arrangement of the 

 tracheal system appeals to be very similar to that 

 described in SynleMes (p. 87). The following 

 account of the Triquetro-quadrate Type is made 

 almost entirely from a study of transverse and 

 longitudinal sections. 



i. The Median or Quadrate Gill (Plate iii,, fig.23; 

 Plate v., fig.34). The cuticle is unpigmented, and 

 considerably thicker than in the saccoid type. This 

 thickening is especially noticeable at the two lateral rj,^" fio is 

 angles of the cross section (mr in Text-fig. 17). 

 Here the total thickness is 15/x or more, of which the hard, 

 outer structure, staining deeply with eosin, may occupy 3/;. or 

 more. The cuticle throughout the portions of the gill lying close 

 above the lateral angles is somewhat thicker than that below them. 



Seen from outside, in the whole mount, the thickened cuticle 

 of the lateral angles forms a kind of mid-rib for the gill, on either 

 side of it. In the distal portion of the gill, the central area 

 between the two midribs stands out clearly thickened, in contrast 

 with the outer portions, which become thin and blade-like. This 

 closely approximates to the condition seen in the median gills of 

 the Vertical Lamellar type, in which I have termed the thickened 

 central portion, with the two midribs, the rachis, the thin outer 

 portions the blade. 



* Gills of Calopttryx sp., lateral view, from a specimen in alcohol; ( x 6). 



