MYTILUS. MONOMYARIA. 87 
posterior side generally truncated, and always gaping ; the 
anterior margin nearly a straight line, and a little open 
in the centre for the passage of the byssus; hinge without 
teeth ; ligament margin greatly lengthened and linear ; liga- 
ment partly internal, and continuing along the whole dorsal 
margin ; two muscular impressions in each valve, the poste- 
rior one very large, almost central, the anterior one terminal, 
and sometimes double ; muscular impressions of the mantle 
destitute of a sinus. 
Pinna lanceolata, Plate VIII. fig. 28, P. margari- 
tacea. Plate IX. fig. 7. 
The shells of this genus cannot be mistaken, their characters being so dis- 
similar to all others. There are some of the more produced Mytili, which 
bear a distant resemblance to them, but these are at once distinguished by 
their basal termination being closed. Some of the Pinne grow to a large 
size, exceeding two feet in length. They are extremely brittle, from their 
fibres being perpendicular, and arranged alongside of each other, and but 
slightly attached. A very thin pearlaceous substance is spread over the inside, 
but hardly extends beyond the muscular impression of the mantle, and merely 
covering that portion of the shell occupied by the animal. 
The Pinne are marine shells, and are found in most seas. Three species 
inhabit the British coasts. 
Several fossil species are met with in the tertiary and secondary strata of 
marine origin. The London Clay contains one species, and one also occurs 
in the Calcaire-grossier of France. : 
Lamarck applies the specific term subquadrivalvis to some of the Pinne, and 
tetragona to another, which seems to have been suggested to him by the 
circumstance of the pearlaceous substance of their interior being longitudinally 
divided, taking its rise at the internal anterior point, and continuing somewhat 
more than half-way towards the centre of the posterior side; an external 
organ corresponding to this is frequently observable ; and such shells easily 
break in the direction of this line. 
Genus XXX VII. — MYTILUS. — Linneus. 
Generic Character. — Shell equivalve, regular, longitu- 
dinal, somewhat wedge-shaped, with the beaks terminating 
in a pointed summit; posterior side rounded, and closed ; 
base forming a continuous line with the interior margin in 
a direction oblique to the hinge line; anterior margin 
gaping slightly in the centre, for the passage of the byssus; 
hinge destitute of teeth.; ligament marginal, linear, greatly 
elongated, and subinternal ; outside covered with a strong 
horny epidermis; two muscular impressions, the posterior 
one large and irregular, the anterior very small, and ter- 
minal ; muscular impressions of the mantle irregular ; 
destitute ofasinus. : 
Mytilus antiquorum Plate VIII. fig. 19. 
