264 MANUAL OF THE MOLLUSCA. 



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



Fossil, 50 sp. Devonian . U. S. Europe, S. India. 



The shell of the Pinna attains a length of two feet ; when young it i 

 thin, brittle, and translucent, consisting almost entirely of prismatic eel] 

 layers ; the pearly lining is thin, divided, and extends less than halfway from 

 the beak. Some fossil Piunas crumble under the touch into their componen 

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

 moored vertically, and often nearly buried in sand, with knife -like edges ered 

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

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

 of the Pinna, was anciently believed to have formed an alliance with the blint 

 shellfish, and received the name of Pinna-guardian (Pinnoteres] from Aris 

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



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

 gene, 3 ' Saussure.) Shell thick, inequi valve, somewhat irregular, margin 

 undulated. Fossil, 5 sp. Oolitic strata of England and France. Fragment 

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

 grown individuals are supposed to have measured a yard across. 



FAMILY III. MYTILID^. Mussels. 



Shell equivalve, oval or elongated, closed, umbones anterior, epidermi 

 thick and dark, often filameutose ; ligament internal, sub-marginal, very long 

 hinge edentulous ; outer shell layer obscurely prismatic-cellular ;* inner mor 

 or less nacreous ; pallia! line simple ; anterior muscular impression small an< 

 narrow, posterior large, obscure. 



Animal marine or fluviatile, attached by a byssus ; mantle-lobes unitec 

 between the siphonal openings ; gills two on each side, elongated, and unitec 

 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 shell-fragments, burrowing in soft substances, 01 

 secreting themselves in the burrows of other shells. 



MYTILUS, 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 nearl; 

 equal ; palpi long and pointed, free. 



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

 epidermis. (Carpenter.) 



