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ORDER II CONCHIFERA UNIMUSCULOSA. 
SHELL presenting internally one muscular impression, nearly in the centre. This or 
tier is divided into Three Sections. 
SECTION I. 
Ligament marginal, elongated on the margin, sublinear. 
This section contains Three Families — Tridacnea, Mytilacea, and Mallacea. 
TRIDACNEA 2 Genera. 
Shell transverse, equivalve, the muscular impression under the middle of the superior margin, and 
is prolonged to each side under it. 
Tridacna. — Shell regular, equivalve, inequilateral, transverse, gaping at the lunula: 
hinge with two compressed, unequal, anterior, entering teeth. Ligament mar- 
ginal, external. 
Plate X. Fig-. 6. T. squamosa. 
(a) Inside of ditto. 
Hippopus. — Shell equivalve, inequilateral, transverse; the lunula close: hinge with 
two compressed, unequal, anterior, and entering teeth. Ligament marginal, 
external. 
Plate X. Fig. 7. H. maculatus. (Chama hippopus. — Linn.) 
MYTILACEA — 3 Genera. 
1 he ligament at the hinge suhinternal, marginal, linear, very entire, occupying a great part of the 
anterior margin ; rarely foliated. 
Modiola. Shell subtransverse, equivalve, regular, the posterior side very short; beaks 
nearly lateral, depressed on the short side: hinge without teeth, lateral, linear. 
Ligament cardinal, almost internal, situated in a marginal canal : one sublateral 
muscular impression, elongated, and axe-shaped. 
Plate XI. Fig. 1. M. tulipa. (Mytilus modiolus. — Linn.) 
Mytilus. — Shell longitudinal, equivalve, regular, pointed at the base, fixed by a byssus; 
beaks almost straight, terminal, pointed : hinge lateral, mostly without teeth. 
Ligament marginal, subinternal: one elongated, club-shaped, sublateral, muscular 
impression. 
(1) Shells with longitudinal furrows. 
Plate XI. Fig. 2. M. erosus. 
(2) Shells without longitudinal furrows, 
Plate XI. Fig. 3. M. perna. 
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