18 
ORDER II CONCHIFERA UNIMUSCULOSA. 
SHELL presenting internally one muscular impression, nearly in the centre. This Ol- 
der 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. 
The ligament at the hinge subinternal, 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. 
