420 MANUAL OF THE MOLLXJSCA. 



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 

 {Pinrioteres) iroTCi Aristotle; similar species infest the Mussels 

 and Anomice of the British coast. 



Sub-genus, Trichites (Plott), Lycett. T. Plottii, Llhwyd. 

 (" Pinnigene," Saussure.) Shell thick, inequivalve, somewhat 

 irregular, margins undulated. Fossil, 5 species. Oolitic strata 

 of England and France. Fragments an inch or more in thick- 

 ness are common in the Cotteswold-hills ; full-grown individuals 

 are sujDposed to have measured a yard across. 



Family III. — Mytilidje. 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 fluviatile, 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 laminae free ; 

 foot cylindrical, grooved. 



The members of this family exhibit a propensity for conceal- 

 m.ent, frequently spinning a nest of sand and shell-fragments, 

 burrowing in soft substances, or secreting themselves in the 

 burrows of other shells. 



Mytlltjs, L. Sea-mussel. 



Example, M. smaragdinus, PI. XVII., 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. 



Aninud wiih. 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 

 uncovered at low- water ; the fry abound in water a few fathoms 

 deep ; they are full-grown in a single year. From some un- 



* A thin layer of minute cells may frequently be detected immediately ur.der the 

 epidermis. (^Carpenter.) 



