170 THIRTY-FIFTH REPORT ON THE STATE MUSEUM. 
All dissections were made by myself, and all the drawings are 
directly from nature by myself; only in two or three instances has the 
microscope been used, nearly all the parts described can be seen with- 
out the use even of a simple lens. 
The Anodonta consists of an outer part called the shell or exo-skeleton, 
and the animal inclosed within the shell. The shell is composed of 
two valves united dorsally by means of an elastic ligament. 
The parts of the animal which will be described in detail are the man- 
tle, gills, labial palpi, muscles, body, foot, viscera, nerves, liver, stomach, 
heart, pericardium, renal organ, vascular system and organs of genera- 
tion. 
When the shell of the Anodonta (and these remarks apply also to the 
genera Unio and Margaritana) is held with the ligament or attached 
portions of the shells upward, and the larger, most convex portion, the 
most distant from the eye, the valve at the right hand is called the 
right valve (Pl. 4, fig. 1, r.v.); that at the left, the left valve (PI. 4, fig. 
1, 1. v.); the part the most distant from the eye, the anteror portion 
(Pl. 4, fig. 1, a. p.); the nearest to the eye, the posterior portion (PI. 
4, fig. 1, p. p.) ; the upper part the dorsal (PI. 3, fig. 1, d. p.), and the 
lower part the;ventral portion. Where the same letters occur on PI. 
3 as on Pl, 4, the same parts of the shell aredesignated. On the dorso- 
anterior portion of each valve is a more or less prominent, blunt 
elevation called the umbo or beak. (Pl. 3, fig. 1, umb.) 
Posterior to the umbones, uniting the dorsal margins of the valves, 
is an elastic horny portion of the exo-skeleton, designated the hgament 
(Pl. 4, fig. 1, lig.), which antagonizes the action of the adductor mus- 
cles, and has a tendency to keep apart the ventral margins of the 
valves. 
The ligament when the shell is.open is of nearly equal thickness; 
when the shell is closed by the action of the adductor muscles, the 
outer portion of the ligament becomes stretched and the inner por- 
tion compressed and folded; when the muscles relax, the ligament 
assumes its natural form and in doing so draws apart the ventral mar- 
gins of the shell. 
The ligament is said to be external, though it is covered for about one- 
- half of its width by an extension of the shell. This ligament is com- 
posed of two parts, the outer and thinner portion the epidermal, 
and the inner portion the cartilagenous, composed of both perpen- 
dicular and horizontal fibres. 
The shell or exo-skeleton consists of three distinct layers, the outer 
one is designated as the epidermis (Pl. 4, fig. 3), and consists of a thin 
membrane which is uncalcified, that is, without lime in its composi- 
tion, and varying in color from olive green to brown, Immediately 
