170 Thirty-fifth Report ox 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 (PI. 4, fig. 1, r.v.); that at the left, the left valve (PL 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 (PL 

 4, fig. 1, p. p.) ; the upper part the dorsal (PL 3, fig. 1, d. p.), and the 

 lower part the ventral portion. Where the same letters occur on PL 

 3 as on PL 4, the same parts of the shell are designated. 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 ligament 

 (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 



