8 PARTS OF BIVALVES EXPLAINED. 
the outside, and sometimes the inside, of both bivalves and 
univalves. When the striz are indistinct, they are said to be 
obsolete. The character and disposition of the striz are of 
much consequence in the discriminations of species. When 
it is both longitudinal and transverse, it is called decussated 
strize. 
Accessory Valves are small testaceous, or shelly plates, 
situated at the back of the hinge of the genus Pholas. 
Muscular Impressions. These are of much use in the 
generic and specific distinctions of bivalves. The most 
conspicuous are those of the ADpuctor MusctEs, by which 
the valves are opened and closed. Fig. 22. g g. and fig. 
23. ¢. 
Pallial or Mantle Impression. In all bivalves in which 
there are two muscular impressions, there is a depressed line 
to which the mantle is attached, extending from the upper 
and inner edge of one muscular impression to the other ; 
this line is frequently deeply sinuated on the posterior side. 
Fig. 22. n. 
Ribs or Coste are longitudinal or transverse protube- 
rances, running from the apex to the base, fig. 20, a a. or 
across the valves from side to side. Plate VII. figs. 17 and 
29 ; and plate VIII. fig. 22. 
Base, that part of the valve opposite to the wmbo. 
PARTS OF THE ORDER CIRRIPEDES. 
The shells of this order are either attached by their base: 
to extraneous bodies, or are seated on a peduncle, which 
adheres to stones, wood, &c. 
Base, is that part by which the shell adheres to other 
substances 
Summit, the opposite extremity to the base. 
Operculum, consists of four or more small valves, which 
close the cavity or opening at the summit of shells of the 
genus Balanus, &c. 
Peduncle or Pedicle is a membranous substance, similar 
to a bladder, on which the genera Pentilasmis, Polycipes, 
&c. are seated. 
All other terms peculiar to the science of Conchology, 
will be found in the glossary at the end of the volume. 
