st. Sense tentacle. 



v. Ventricle. 



va. Visceral arteries. 



vg. Visceral ganglion. 



vl. Velum. 



vm. Visceral mass. 



vv. Visceral veins. 



x. Swelling on the visceral ganglion from which the anterior root 



of the branchial nerve originates. 

 y. Swelling on the visceral ganglion from which the posterior pallial 



nerves originate. 



PLATE i. 

 FIG. i. A well preserved left shell valve showing the markings on the 



Outer surface. Two-thirds natural size. 

 FIG. 2. Outer surface of a left shell valve that shows distinct radial 



color markings. Two-thirds natural size. 



PLATE 2. 



FIG. 3. Left shell valve badly mutilated by the attacks of boring sponges. 

 The large barnacle near the margin of the valve shows that 

 the rate of growth has not been very rapid for some time. 

 Two-thirds natural size. 



FIG. 4. Outer surface of a right shell valve. The valve is flatter than 

 its mate and has a conspicuous notch at the base of the 

 anterior wing. The radiating ridges are worn so they are 

 not as conspicuous as they are on the other valve. This valve 

 is usually lighter in color than the left. The relatively dark 

 color here is due to different printing. The round openings 

 on the surface are due to recent attacks of the boring sponge. 

 Two-thirds natural size. 



PLATE 3- 



FIG. 5. Inside of a left shell valve. The markings on the inside of the 

 left shell valve are never as conspicuous as they are on the 

 inside of the right shell valve. Two-thirds natural size. 



FIG. 6. Inside of a right shell valve. The division of the adductor 

 muscle into a large anterior and a small posterior portion is 

 conspicuously shown by the scar on this valve. Two-thirds 

 natural size. 



PLATE 4- 



FIG. 7. Inside of the right shell valve of a specimen that has been rather 

 badly attacked by boring sponges. The roughenings on the 

 surface seem to cover the deep borings of the sponge. Two- 

 thirds natural size. 



68 



