86 MONOMY ARIA. PINNA. 
the posterior side, near the sinus for the byssus, which 
answer as points of attachment for a part of the mantle. 
Perno maxillata, Plate VIII. fig. 30. 
The elevations between the sulci do not fit into the grooves of the opposite 
valve, when the shell is closed, in the manner as cardinal teeth, but rest 
against the corresponding ones in the opposite valve. 
The front extremity is often protruded in the form of a pointed lobe, but 
this is not a constant generic character. 
The shells of this genus may be confounded with those of Crenatula, but 
that genus is destitute of the sinus through which the byssus passes, and its 
hinge is lateral. The parallel grooves in the Perne contain the. principat 
portion of the ligament, whereas this portion is situate in the crenulations of 
the Crenatulz ; which latter genus consists of much more fragile shells than 
those of Perna. In the Crenatule the pearlaceous interior occupies a much 
smaller portion of the shell, seldom exceeding more than half the dimensions, 
of the valves, extending along the hinge margin, and forming an oblique 
parallel line from the beak to the extreme point of the valve. 
The Perne inhabit the ocean, and are found only in the seas of New 
Holland and India. 
Shells of this genus are also known in a fossil state, and are met with in the 
Calcaire-grossier, the Clunch Clay, the London Clay, and in the Oolitic group 
of rocks. 
Genus XXXV. — PLUVINITES. — Defrance. 
Generic Character. — Shell inequivalve, very inequi- 
lateral, compressed, and thin ; anterior side gaping a little; 
one valve much flattened ; the other a little convex ; hinge 
linear, short, situate immediately behind the umbones, and 
divided by perpendicular grooves; each valve furnished 
with two small muscular impressions, one of which is very 
minute, and placed close below the hinge, the larger one 
lower down, and almost central; ligament supposed to be 
internal. 
Pluvinites Adansoni. Plate IX. figs. 5, 6. 
This shell, the anly one of the genus, is closely allied to both Crenatula and 
Catillus, and is known only in a fossil state. It is found in a stratum of 
Baculite limestone, near Fresville, at Volognes, Normandy. 
TRIBE II. — MYTILACEA. 
Hinge, with the ligament sub-interior, marginal, linear, 
very entire, occupying a great portion of the anterior border ; 
shell rather foliaceous. ; 
Genus XXXVI. — PINNA. — Linneus. 
Generic Character. — Shell equivalve, longitudinal, 
oblique, wedge-shaped ; beaks forming an elongated point; 
