Il8 ALUS. [Vol. XVIII. 



being placed longitudinally in the gill, and hence transversely to 

 the width of the filament. On the four branchial arches there are 

 two rows of these rays, corresponding to the two rows of fila- 

 ments, the V-shaped ends of the rays of each row resting longitu- 

 dinally upon the corresponding edge of the related bony element 

 of the arch. Distal to its V-shaped end each gill ray is formed of 

 a median cylindrical portion, or shaft, bordered on each side, for 

 some distance from its base, by a thinner, flattened portion, the 

 distal edge of which is approximately parallel to the corresponding 

 part of the V at its proximal end. Each ray thus has the shape 

 and appearance of a feathered arrow, the feathers of the arrow 

 lying in the plane of the arch, and the notch at its proximal end 

 lying transversely to the feathers. The rays of the two rows of 

 filaments on each arch are placed alternately to each other, the 

 rays of one row lying opposite the spaces between the rays of the 

 other. Each ray gives insertion on its inner surface, beyond its 

 feathered portion, to a delicate, spindle-shaped muscle, which 

 arises, by a long, double-headed tendon, from the outer surfaces 

 of the two rays that lie opposite it and adjoin each other in the 

 other row of filaments, the surfaces of origin lying immediately 

 beyond the V-shaped ends of the rays. The surface of insertion of 

 the tendon is relatively long, and the gill filaments are, naturally, 

 free, or detached from each other, only in that part that lies distal 

 to this surface ; that is, for about two thirds their full length. By 

 the contraction of these muscles, the filaments of one row are made 

 to approach those of the other, and even to pass partly between 

 them, with a scissor-like movement. On the demibranch no trace 

 of these muscles could be found. 



On both sides of each of the first four arches, toward the oral 

 surface of the arch, there is a row of flat, bony processes of der- 

 mal origin. These processes (Figs. 38-42), on the antero-lateral 

 surface of the first arch, are long and pointed, but elsewhere have 

 the short, blunt form shown in Fig. 41. On the antero-lateral 

 surface of the first arch they project forward, or forward and 

 orally. On the antero-lateral surfaces of all the other arches they 

 project almost directly laterally. On the postero-mesial surfaces 

 of all the arches they project almost directly backward. The oral 

 surfaces of all the arches are reduced to a narrow line, and the 



