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PINNIDA. 283 
Famity PINNIDA. 
Pinna, Linn, 1758. 
Etym.—Pinna, a fin or wing. 
Distr.—30 sp. U.S., Britain, Mediterranean, Australia, 
Pacific, Panama. Fossil, 60 sp. Devonian—. Increasing to 
the present time. U.S., Europe, South India. P. rudis, Linn. 
(GXxxt, 67). 
Shell equivalve, wedge-shaped ; umbones quite anterior; pos- 
terior side truncated and gaping; ligamental groove linear, 
elongated; hinge edentulous; anterior adductor scar apical, 
posterior subcentral, large, ill-defined; pedal scar in front of 
posterior adductor. 
Animal with the mantle doubly fringed; foot elongated, 
grooved, spinning a powerful byssus, attached by large triple 
muscles to the centre of each valve; adductors both large; palpi 
elongated ; gills long. 
The shell of the Pinna 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 half-way from the beak. Some fossil Pinnas 
crumble under the touch into their component fibres. The living 
species range from extreme low-water to sixty fathoms; they 
are moored vertically, and often nearly buried in sand, with 
knife-like edges erect. The byssus has sometimes been mixed 
with silk, spun, and knitted into gloves, ete. 
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 Anomize 
of the British coast. 
ATRINA, Gray, 1840. Shell irregular, valves connate,as though 
soldered together on the dorsal margin. /P. saccata, Linn. 
(cxxxi, 68). 
PALZOPINNA, Hall, 1883. Shell gaping in front; surface marked 
by fine radiating lines. More convex and with finer rays than 
in Pinna. 2sp. Paleozoic; N. Y. P. recurva, Hall. 
TRICHITES, Defrance, 1828. (Pinnigena, Agassiz, 1847.) Shell 
thick, inequivalve, somewhat irregular, margins undulated. 
Fossil, 5 sp. Oolitic strata of England and France. P. undatus, 
Lycett (cxxxii, 88). Fragments an inch or more in thickness 
are common in the Cotteswold-hills ; full-grown individuals are 
supposed to have measured a yard across. 
AVICULOPINNA, Meek, 1864. Very elongately subtrigonal, 
eyuivalve, with slightly indicated subterminal beaks, the shell 
being somewhat produced in front of them, posteriorly gaping ; 
