i6 



JEAN DAWSON. 



mm. in length. The gill lamellae do not reach the external open- 

 ing hence there is left a smooth space about the opening. This 

 space is much wider than the one found about the internal gill 

 opening and extends farther along the dorsal and ventral borders 

 (Figs. 9 and 10). 



The lamellae are attached by one margin along their whole 

 length except near the external gill opening. Here they are 

 broadened abruptly and their ends are free. These enlarged free 

 ends of the lamellae are somewhat triangular and are bent dor- 

 sad or ventrad so that they overlap each other along the edge of 



b c a d 



FIG. 9. A portion of the interior of the gill sac showing the external branchio- 

 pore whh its ental valves in the position they assume when water is entering the gill 

 sac. a, ental valve ; b, central lamella; which splits and forms the entai valves ; c, 

 laminae ending free ; d, cartilaginous ring of the external branchiopore. 



the distal smooth area (Figs. 9 and 1 1). The central lamella on 

 both the cephalic and caudal walls splits into two laminae about 

 10 mm. from its free ends (Figs. 9 and 1 1, b}. The laminae of the 

 cephalic wall end free. When those of the caudal wall reach 

 the edge of the cartilage which surrounds the external gill open- 

 ing, there extends from each a membranous sheet which forms 

 one of the ental valves (Fig. 9, a ; Fig. 10, a}. 



Each ental valve is a nearly circular concave plate attached by 

 less than one third of its circumference to the caudal half of the 

 medial end of the external branchiopore. The branchiopore 



