264 MANUAL OF THE MOLLUSCA. 



Distr. 30 sp. U. States, S. Brit. Medit. Australia, Pacific, Panama. 



FossU, 50 sp. Devonian — .' U. S. Europe, S. India. 



The shell of the P'mna attains a length of two feet ; when young it is 

 thin, brittle, and translucent, consisting almost entirely of prismatic cell- 

 layers ; the pearly lining is thin, divided, and extends less than halfway fi-om 

 the beak. Some fossil Pinnas crumble under the touch into their component 

 fibres. The living sp. range from extreme low- water to 60 fms ; they are 

 moored vertically, and often neai'ly buried in sand, with knife -like edges erect. 

 The byssus has sometimes been mixed with silk, spun, and knitted into 

 gloves, &c. (Brit. Mus.) A little crab which nestles in the mantle and gills 

 of the Pinna, was anciently believed to have formed an alliance mth the blind 

 shellfish, and received the name of Pinna-guardian {Pinnoteres) from Aris- 

 totle ; similar species infest the Mussels and Anomim of the British coast. 



Suh-genus, Tnchites, (Plott) Lycett. T. Plottii, Llhwyd. ("Pinni- 

 gene,'' Saussure.) Shell thick, inequivalve, somewhat irregular, margins 

 undulated. Jbwz'/, 5 sp. Oolitic strata of England and Prance. Fragments 

 an iucli or more in thickness are common in the Cotteswolde-hills ; full- 

 grown individuals are supposed to have measm'ed a yard across. 



FAMILY III. MiTiLiD^. 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 ; giUs two on each side, elongated, and united 

 behind to each other and to the mantle, dorsal margins of the outer and inner- 

 most laminae free ; foot cylindrical, grooved. 



The shells of this family exhibit a propensity for concealment, frequently 

 spinning a nest of sand and sheU-fragments, bui-rowing in soft substances, or 

 secreting themselves in the burrows of other shells. 



Mttilus, L. Sea-mussel. 



Ex. M. smaragdinus, PI. XVII. fig. 4. 



Shell wedge-shaped, rounded behind ; umbones terminal, pointed ; hinge- 

 teeth minute or obsolete ; pedal muscular impressions 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. 



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

 epidermis. {Carpenter.) 



