CONTENTS OF VOL. II. xli 



Page. 



Subdivision I. Cloak open. 



Tribe I. Valves closed by one adductor muscle. 

 Pectenidae. Ostreadae, . . .504. 



Tribe II. Shell closed by two adductor muscles. 

 Avicularia. Meleagrina. Pinna. Arcadas, . 505 



Subdivision II. Cloak more or less closed, forming sy- 

 phons. 



Tribe I. 



One syphon. Mytilusidae. Uniodae. Cardita. Vene- 



ricardia and Crassatella, . 505 



Tribe II. 



Cloak closed posteriorly, and anteriorly, forming three 



apertures. Tridacna and Hippopus, 506 



Tribe III. 



Anterior opening large. Chama. Cardium. Donax. 



Tellina, Venus. Mactra, . . 506 



Tribe IV. 



Anterior opening small. Mya. Solen. Pholas. Teredo, 507 



Section II. ACEPHALA TUNTCATA. 



Subdivision I. Interior tunic detached from the external 

 one, and united only at the two orifices. 



Tribe I. 



Body permanently fixed to other bodies. 



A. Simple animals. 



1 . Apertures furnished with four rays. Boltenia. Cy- 



nthia. Caesira. Styela. Pandocia. 509 



2. Apertures with indistinct rays of more than four. 

 Clavelina. Pirena. Ciona, Phallusia 511 



B. Compound animals. 

 Branchial orifice radiated. 



a. Branchial and anal orifices, with six rays. 



Diazona. Polyzona. Sigillina. . .512 



