PLATE XL 



Fig. 1. — Interior of shell, 

 a. a. Impression of anterior ad- p. r. Impression of posterior re- 



ductor muscle. tractor muscle, 



p. a. Impression of posterior ad- p. 1. Impression of pallial line. 



ductor muscle. m. f. Impression of umbonal mua- 



a. r. Impression of anterior re- cles. 



tractor muscle. p. 1. m. Pallial line muscles, 



p. p. Impression of protractor r. t. Rudimentary teeth. 



pedis muscle. mar. Markings left by the adduc- 



tor muscles. 

 Fig. 2. — Interior of shell of TJnio complanatus. 

 a. a. Anterior adductor. p. r. Posterior retractor, 



a. r. Anterior retractor. p. 1. Pallial line, 



p. p. Protractor pedis. c. t. Cardinal teeth. 



p. a. Posterior adductor. lat. Lateral teeth. 



Fig. 3. — The mantle thrown back, showing the labial palpi (1. p.) in position. 

 Fig. 4. — Transverse section, 

 sh. Shell. o. b. Organ of Bojanus. 



m. Mantle. v. Ventricle, 



o. g. Outer gill. au. Auricle, 



i. g. Inner gill. p. Pericardium, 



f. Foot. r. Rectum. 



