318 AMERICAN JOURNAL 



operculum in contact with the labrum is generally more or less de- 

 tached from the foot; it is also much thinner and more elastic at 

 this point, an arrangement conducing to greater flexibility, and, 

 as a consequence, the animal is enabled to retire farther within 

 the aperture where the closure becomes complete. 



In Leptoxis Foremanii, Lea, that portion of the operculum in 

 contact with the labrum presents a beautiful, serrated appear- 

 ance (plate 21, fig. 4), and is the only one of this character ob- 

 served in the examination of a wide range of species. The color 

 of the opercul^ of the Strepomatid(c, when viewed by transmitted 

 light, is usually a dark reddish brown, while a few are of a pale 

 brown or a light yellow color. In the adult operculum, beside the 

 whole inner surface being strengthened by a general deposit from 

 the operculigerous lobe, there are often two or three bands running 

 from the polar point towards the superior angle, and some of the 

 larger species of Goniohasis and Scliizostoma present, on their 

 inner surface, fasciculi of irregular, radiating striae, which seem- 

 ingly strengthen the attachment of this organ to the operculi- 

 gerous lobe. 



In foreign 3Ielanida> the growth and structure of the opercu- 

 lum is similar to the Strepomatidce, except that it is more finely 

 laminnte and the spiral character is always present, a condition 

 probably owing to their inhabiting still water, while the Strepo- 

 matids are generally denizens of rapid and rocky streams. So 

 far as I have examined, with one exception,* the number of 

 spiral turns in the operculum of native and foreign 3Ielanic7is bear 

 no relation to the number of whirls in the shell, the deposit of 

 the semi-concentric plates being seemingly limited only by the 

 growth of the animal and the necessity for a perfect closure of 

 the aperture of the shell. 



The spiral character of the operculum indicates the design 

 (so far as it relates to the external covering in this family) to 

 carry out the bi-lateral form. 



When the animal retires within the shell the posterior half of 

 the body is folded on the anterior, resembling, in its action, the 

 shutting of a clasp-knife, after which the head and body is with- 

 drawn, leaving the operculum, which is situated on the end of the 

 tail, to close the aperture. 



* In Mtlania Largillierti, Phil., the number of turns in the shell and 

 operculum correspond, being seven in each. 



