BY R. J. TILLYARD. 83 



Tnternally, the structure of the triquetral gill closely resembles 

 that of a lateral saccoid gill, except for the shape. There is a 

 very great development of the alveolar mesh work. The zones 

 of principal internal laminae are arranged in such a way as to 

 cut off, in cross section, the upper and the lower inner corners 

 of the triangle, the dorsal laminae lying almost horizontally, and 

 the ventral ones not far from vertically. This arrangement, 

 again, is paralleled in the obliquity of these laminae in the lateral 

 gills of the saccoid type. 



In the central part of the gill, the two blood-canals, the two 

 main longitudinal tracheae, and the two main nerves, run un- 

 symmetricaljy, the ventral set being placed at a lower and inner 

 level compared with the dorsal. A very peculiar development, 

 confined, as far as I can ascertain, to gills of this type, is the 

 splitting-up of the blood-canals, each of which gives off an im- 

 portant longitudinal branch to run in the outer portion of the 

 gill, on the other side of the' principal internal laminae (Text-fig. 

 17, dh, vh). All four branches become confluent again near the 

 distal end of the gill. 



Secondary internal laminae are developed at frequent intervals, 

 within the two portions of the gill cut off by the principal 

 lamime. These lamina3 lie parallel with the corresponding 

 principal laminae, but not all at the same level, so that sometimes 

 two or more of them may be cut in the same section. 



C. The Lamellar Type. 



(Text-figs.19-29; Plate i., figs.1-5, 7-11; Plate ii., figs.12-20; 

 Plate iii., figs.25-27; Plates v. and vi., figs.36-39. 



In this type, all three gills are reduced to flattened lamellar 

 organs, with consequently a great reduction in the volume of 

 the interior of the gill. The alveolar meshwork becomes much 

 reduced in amount, and is principally confined to the thickened 

 portion of the gill, lying beneath the mid-rib. This thickened 

 central portion, I propose to term the rachis, using the term 

 hlade for the outer and flatter portions of the lamella. 



The rachis is always bounded, either on one or both sides, by 

 a thickened ridge of cuticle, forming a strong mid-rib (mr). In 

 the interior of the rachis run the longitudinal tracheal trunks, 



