1869.] On the genus Onchiduim. 103 



mantle is below blackish, with a grey or brown tint, pale at the 

 margins ; the foot is greenish yellow, the dark colour of the digestive 

 and the pale reddish colour of the generative organs shining through 

 the skin. The width of the foot, which is little shorter than the 

 mantle, amounts to about -|ths of the width of the latter, but when 

 the animal creeps about, it may be estimated at f th of that width ; it is 

 truncate in front and rounded posteriorly. The anus lies at the upper 

 basal end of the foot, the opening being small and not distinct, cover- 

 ed by the terminal free edge of the foot. The pulmonary orifice is 

 situated immediately beyond the anus, its internal margin is smooth. 

 The hermaphrodite genital pore is a longitudinal slit, surrounded by 

 swollen lips, situated about y^th of an inch distant to the right of the 

 anus. The external vas deferens, in the fold between the foot and the 

 mantle, is marked as a white groove, and terminates in a minute pore 

 below the right buccal appendage. The male genital pore lies in front, 

 below the right pedicle. The dentition has been described previously (see 

 p. 91, pi. xiv, fig. 6a). 



The length of large specimens is about 2|- inches, and the width 

 varies from one-third to one-fourth of it, when the animal moves about 

 in its ordinary way. The usual length of pedicles is about half an inch. 

 Old specimens, when fresh caught, very often secrete from the smooth 

 lower portion of the mantle, a deep carmine red, gelatinous substance, 

 of a distinct alkaline character. The substance coagulates in spirit, 

 but is partially dissolved by, or is at least made thinner in, glycerine. 



I have already mentioned, that this species is the commonest, and 

 as yet the only one which was found near Calcutta. It is seen crawling 

 about on old bricks, in ditches on the maidan, about the fort, along the 

 Tollis-nullah (canal), and locally also on the banks of the Hooghly. 



2.— Onchidium pallidum, Stol, Pi. xv, Fig. l. 



Body elongated, moderately elevated, rounded anteriorly and pos- 

 teriorly, generally covered with copious mucus. The mantle above 

 is pale yellowish white, with a central, blackish, longitudinal stripe, 

 commencing above the head, and extending posteriorly to about jth of 

 the length of the body. It is accompanied on either side by a pale 

 yellowish or greyish stripe, and the interspaces between these and the 

 central stripe, are somewhat darker than the general colour of the 



