PLATE 13. 



Fig. 15.— A piece of branchial membrane from a small specimen (1.5 

 mm. by 1.25 mm. i. from the apace between two folds and near the endostyle. 

 All details of this and of figs. 16 and 17 were drawn with the greatest 

 possible accuracy. 



16. — See Plate 14. 



pig. 17.— A membrane in which the stigmata are on the whole not quite 

 so long as those of fig. 16, though the difference is hardly recognizable in 

 the figure. The more incomplete state of S' ; in this membrane than in 

 the one shown in 16 will be noticed. 



[112] 



