EXPLANATION OF PLATES. Ho 
retracting muscle of the respiratory tubes may be seen in their 
natural positions. The umbonic portion of the right valve, 
which remains, shows the large size of the perforations destined 
for the attachment of the branches of the anterior adductor 
muscle. 
Tr. The reflection of the shell over the umbones in Figs. 1 and 2. 
‘ig. 2. This Figure represents the same animal, the right valve and 
mantle beg entirely removed, and the respiratory tubes split 
vertically, their left sides alone remaining. 
The lips, the foot, the pointed abdomen, the position occupied by 
the branchiz, the cells behind the branchiz, and the course of the 
orbicular muscle, may be distinctly seen. 
Fig. 3. Gastrochzna Faba, viewed lying on its left side, its respira- 
tory tubes being fully exserted. The very small aperture for 
the exit of its foot is seen between the anterior and posterior 
junctions of the mantle. 
Fig. 4. Mya arenaria, in nearly the same view as Pholas Dactylus 
in Fig. 2; the mght orbicular muscle still remaining. The 
aperture for the exit of the foot is small and narrow, situated in 
the same position as in Fig. 3. 
For all the letters referring to the parts of the shells and their 
animals in this Plate, see the preceding Explanation. 
PLATE IV. 
Fig. 1. Callista verrucosa, viewed from above, showing the slightness 
| of the inequality of its valves, is intended to explain the names 
of its external parts and characters. 
Fig. 2. Inner view of shell of Callista verrucosa, showing the mus- 
cular impressions: the specimen here figured exhibits the im- 
pression of the retractor of the respiratory tubes, much longer 
than is generally observed to exist in this species. 
Fig. 3. Cardium edule. The animal in its left valve; the right 
valve and the right mandible being removed. This Figure 
exhibits the anterior and posterior cardinal teeth projecting 
through the mantle; the part of the abdomen which fills the 
right umbo; andthe respiratory tubes, which are vertically divided 
to render visible a little valve within the upper one. The situa- 
tion of the termination of the intestinal canal, above and behind 
the posterior adductor muscle, is seen distinctly. 
Fig. 4. The muscular system of Mytilus pellucidus. The anterior 
and posterior adductor muscle and the posterior adductor liga- 
