\ CONCHIFERA. 419 
TNOCERAMUS, Sowerby (1814). 
Etymology, is (inos), fibre, keramos, shell. 
Example, I. sulcatus, Pl. XVII., Fig. 3. 
Synonym, Catillus, Brongn. 
Shell inequivalye, ventricose, radiately or concentrically 
furrowed, umbones prominent; hinge-line straight, elongated ; 
cartilage pits transverse, numerous, close-set. 
This genus differs from Perna chiefly in form. J. involutus 
has the left valve spiral, the right opercular. J. Cuvieri attains 
the length of a yard. Large flat fragments are common both 
in the chalk and flints, and are often perforated by the Cliona. 
Hemispherical pearls haye been found developed from their 
inner surface, and spherical pearls of the same prismatic-cellular 
structure occur detached, in the chalk. (Wetherell.) The Jno- 
cerami of the gault are nacreous. 
Fossil, 75 species. ? Silurian — Chalk. South America, 
United States, Europe, Algeria, Thibet. 
Pinna, L. 
Etymology, pinna, a fin or wing. 
Type, P. squamosa, Pl. XVI., Fig. 23. 
Shell equivalve, wedge-shaped; umbones quite anterior; 
posterior side truncated and gaping; ligamental groove linear, 
elongated; hinge edentulous; anterior adductor scar apical, 
posterior sub- central, 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. 
Distribution, 30 species. United States, Britain, Mediter- 
ranean, Australia, Pacific, Panama. 
Fossil, 60 species. Devonian —. United States, Europe, 
South India. 
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 
60 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, &c. (Brit. 
