420 MANUAL OF THE MOLLXTSCA. 



Mus.) A little crab whicli nestles in tlie mantle and gills of the 

 Pinna, was anciently believed to bave formed an alliance witb. 

 tbe blind sbell-fisb, and received tbe name of Pinna-guardian 

 {Pinnoteres) from Aristotle ; similar species infest tbe Mussels 

 and Anomice of tbe Britisb coast. 



8ub~genus, Trichites (Plott), Lycett. T. Plottii, Llbwyd. 

 (" Pinnigene," Saussure.) Shell tbick, inequivalve, somewbat 

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

 of England and Prance. Fragments an incb or more in tbick- 

 ness are common in tbe Cotteswold-bills ; full-grown individuals 

 are supposed to bave measured a yard across. 



Family III. — Mttilidjs. Mussels. 



Sliell equivalve, oval or elongated, closed, umbones anterior, 

 epidermis tbick and dark, often filamentose ; ligament internal, 

 sub -marginal, very long ; binge edentulous ; outer sbell 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, attacbed by a byssus ; mantle- 

 lobes united between tbe sipbonal openings ; gills two on eacb 

 side elongated, and united bebind to eacb otber and to tbe 

 mantle, dorsal margins of tbe outer and innermost laminae free ; 

 foot cylindrical, grooved. 



Tbe members of tbis family exbibit a propensity for conceal- 

 ment, frequently spinning a nest of sand and sbell-fragments, 

 burrowing in soft substances, or secreting tbemselves in tbe 

 burrows of otber sbells. 



Mytiltjs, L. Sea-mussel. 



Example, M. smaragdinus, PI. XYIL, Pig. 4. 



Shell wedge-sbaped, rounded bebind; umbones terminal, 

 pointed ; binge-teetb minute or obsolete ; pedal muscular im- 

 pressions two in eacb valve, small, simple, close to tbe adductors. 



Animal witb tbe mantle- margins plain in tbe anal region, 

 and projecting sligbtly; brancbial margins fringed; byssus 

 strong and coarse ; gills nearly equal ; palpi long and pointed, 

 free. 



Tbe common edible mussel frequents mud-banks wbicb are/ 

 uncovered at low-water ; tbe fry abound in water a few fatbom^ 

 deep ; tbey are full-grown in a single year. Prom some unj 



* A thin layer of minute cells may frequently be detected immediately under 

 epidermis. (Carpenter.) 



