374 EXPLANATION OF PLATES. 
ment: two longitudinal muscles are attached by their extremities 
to the valves, near to the msertion of the adductor muscles, for 
retracting the foot and supporting the abdomen: these muscles 
are always more or less confluent towards the middle of their 
course; and in the genera Mytilus and Modiola they give off 
six or seven lateral branches on each side, which are attached 
to the valves behind or at the middle of the shell. The byssus, 
terminated by suckers, arises from the tubercle at the posterior 
base of the foot, and is composed of tendinous fibres. 
Fig. 5. Mytilus pellucidus, the right valve and mantle bemg removed. 
Below the cardinal ligament are certain perforations in the 
shell, whose use is not known. The dotted line shows where 
the right mantle was cut off. In this pomt of view, the lateral 
branches of the left mantle, which supports the abdomen and 
retracts the foot, are visible in their natural position. 
Fig. 6. The back view of Unio ovalis, the dorsal portion beimg re- 
moved, to show the situation of the liver, the heart, and the 
intestinum rectum. 
L.r. in Figs. 3 and 4 signifies the lateral ridge of the skull. 
For all the letters referring to the parts of the animals and their 
shells in this Plate, see the preceding Explanation. 
PLATE V*. 
Fig. 1. Represents the Pecten maximus, or great Scallop; its right 
valve and the right mantle being removed from that part indi- 
cated by the dotted line. 
The superior margin of the shell, ¢. mr. s., is angulated anteriorly 
and posteriorly : these angulated parts are named auricles or ear- 
like processes. O75 represents two protuberances, which it has in 
common with Pecten opercularis and most of its foreign congeners, 
in one or both valves. 61s the part named by Poli Viscera testacea : 
the uses of both these parts are unknown. 
Fig. 2. Represents a vertical section of the Ostrea edulis, or common 
oyster. The dotted part represents that part of the intestinal 
canal which was removed from the right side of the animal to 
show other parts. The mouth, os; the lobes of the lips, Jd. ; 
the oesophagus, 7. @.; the stomach, 7. v.; from which the intes- 
tinal canal is continued, which, after turning twice, it ascends 
above the posterior adductor muscle, m. a. p., and the internal 
cardinal ligament, /. a. p., and terminates in the intestmum rec- 
tum and the anus, 7.r.a., all shown in this Figure; and also 
the liver, h.; cells, 6.c., behind the branchie or gills, 6., over 
