420 MANUAL OF THE MOLLUSCA. 
Mus.) A little crab which nestles in the mantle and gills of the 
Pinna, was anciently believed to have formed an alliance with 
the blind shell-fish, and received the name of Pinna-guardian 
(Pinnoteres) from Aristotle; similar species infest the Mussels 
and Anomice of the British coast. 
Sub-genus, Trichites (Plott), Lycett. T. Plotti, Llhwyd. 
(‘‘ Pinnigene,” Saussure.) Shell thick, inequivalve, somewhat 
irregular, margins undulated. J ossil,5 species. Oolitic strata 
of Kngland and France. Fragments an inch or more in thick- 
ness are common in the Cotteswold-hills; full-grown individuals 
are supposed to have measured a yard across. 
Famity II1.—Mytinipa. Mussels. 
Shell equivalve, oval or elongated, closed, umbones anterior, 
epidermis thick and dark, often filamentose; ligament internal, 
sub-marginal, very long; hinge edentulous; outer shell layer 
obscurely prismatic-cellular ;* inner more or less nacreous; 
pallial line simple; anterior muscular impression small and 
narrow, posterior large, obscure. 
Animal marine or fluyiatile, attached by a byssus; mantle- 
lobes united between the siphonal openings; gills two on each 
side, elongated, and united behind to each other and to the 
mantle, dorsal margins of the outer and innermost lamine free ; 
foot cylindrical, grooved. 
The members of this family exhibit a propensity for conceal- 
ment, frequently spinning a nest of sand and shell-fragments, 
burrowing in soft substances, or secreting themselves in the 
burrows of other shells. 
Mytitus, L. Sea-mussel. 
Example, M. smaragdinus, Pl. XVIL., Fig. 4. 
Shell wedge-shaped, rounded behind; umbones terminal, 
pointed ; hinge-teeth minute or obsolete ; pedal muscular im- 
pressions two in each valve, small, simple, close to the adductors. 
Animal with the mantle-margins plain in the anal region, 
and projecting slightly; branchial margins fringed; byssus 
strong and coarse; gills nearly equal; palpi long and pointed, 
free. 
The common edible mussel frequents mud-banks which are 
uncoyered at low-water; the fry abound in water a few fathoms 
deep; they are full-grown in asingle year. From some un- 
* A thin layer of minute cells may frequently be detected immediately under the 
epidermis. {Carpenter.) 
