Figure 1. Anomia simplex, oblique view; A, the very convex upper valve; 



B, the lower valve; C, the circular hole for a plug which is used to fasten the 

 shell. 



2. Mytilis edulus ; the whole exterior surface of the left valve showing the 

 distinct concentric lines of growth ; the interior makings and teeth of the an- 

 terior end of the right valve are also shown. 



3. Area pexata ; H, the entire (/. e., without sinus), pallial line ; C, the num- 

 erous, transverse, or cardinal teeth of the hinge. 



4. Pecten irradians ; surface covered with radiating ribs ; A and B, the 

 ears making the hinge line straight. 



5. Venus mercenaria, left valve ; A, anterior end ; B, posterior end ; C, dor- 

 sal side ; D, ventral side ; E, umbo ; distance A B, the length of the shell ; C 

 D, the height of shell ; ratio A G to A B, the position of umbo, in this case, 

 l-. Fig. 6, anterior view ; A B, the width of the shell ; D, external ligament; 



C, lunule in front of the umbones. Fig. 7, interior view of the right valve ; 

 C, cardinal teeth ; D, lateral teeth ; E, anterior adductor muscle impression ; 

 G, posterior adductor muscle imi)ression; H, pallial line; I, sinus of the 

 pallial line ; the ventral edge is crenulated. 



8. Mactra solidissima, interior view of right valve ; G, spoon-shaped cavity 

 for an internal cartilage ; D and E, striated lateral teeth ; BAG, angle of the 

 slopes of the shell, 



9, Mya arenaria, oblique interior view of the left valve to show the project- 

 ing cartilage process C; I, the very deep and large sinus of the pallial line H, 

 Fig. 10, dorsal view to show the wide gaping of the ends while the ventral 

 edge is closed. Fig. 11, the expanded animal parts ; D, the united siphons 

 at the posterior end; E, the incurrent oritice; F, the excurrent orifice; G, 

 the foot at the anterior end. B, marks the position of the sbell border called 

 the podium, and C, the position, often angulated, called the gonium. The 

 portion A B, in many shells has either ridges or grooves ; such ridges or 

 grooves are called podial ones; ridges or grooves along the line A C are 

 called gonial ones. These terms are quite useful in the descriptions of Unios 

 and many other bivalve shells. 



12. Donax fossor; external view of left valve and the extended animal parts. 

 This shell is peculiar in having the ligament on the shorter end of the shell ; 

 the longer end being the anterior or foot end. The siphons are here short 

 and divided. Fig. 13, dorsal view, showing the closed ends and the short ex- 

 ternal ligament. Fig. 14, internal view of the right valve. 



15. Petricola pholadiformis; oblique view to show the united mantle below. 

 The siphons are long and separated except at the base; the incurrent orifice 

 E is fringed. 



16. Pholas costata ; anterior end, showing the reflected hinge plates at A, 

 and the widely gaping ends. Fig. 17, dorsal view, showing the two extra 

 valves at D, one in front of the other, over the reflexed hinge plates; A, 

 anterior end ; B, posterior end. 



18. Teredo navalis, showing the animal parts all extended. Fig. 19, the 

 pallets of the posterior or siphon end. Fig. 20, the left valve ; the right is 

 shown on Fig. 18, just below. 



Figures 1 to 3, 5 to 7, and 9 to 11, are about half scale ; figs. 4, 8, 16, and 17 

 are about a third or fourth scale; figs. 12 to 14, and 18 to 20, are about natund 

 size. 



Figure 18 is adapted from A, E, Verrill in Sea Fisheries, and 19 and 20 are 

 taken from Binney Gould's Invertebrates of Mass. The rest are origina'. 



