08 On the genus Onchidium. [No. 2, 



outer edges of these. Both the tentacles and the front edges of the 

 appendages are, as a rule, of a yellowish green colour and somewhat 

 thickened ; the former more so, heing provided with numerous nerves, 

 which issue directly from the anterior edge of the central ganglion, 

 lying at the base of the head, and are a portion of those nerves which 

 supply the lips. During the motion of the animal the tentacles 

 are always moved in front of the edges of the buccal appendages, and 

 when each of them are successively touched with a solid object, it will 

 be observed, that the animal much easier responds to the former than 

 to the latter ; the first being the more sensitive organ. 

 Habits. 

 Dr. Buchannan says that he found Onchidium typha always on 

 Typha elephanlina. This plant is at present not nearly so common 

 as the allied species, Typha angustifolia. However, that is no proof 

 that both the species were formerly not more common than they are at 

 present. No doubt, seventy years ago, swampy grounds, over- 

 grown with vegetation, were more extensive about Calcutta, than they 

 are now when our worthy municipality takes such good care to clear 

 everything away ! In places, however, (along the Eastern Bengal 

 and the South Eastern and Calcutta railway lines, and in Alipore) 

 where both species of Typha grow abundantly I have not been suc- 

 cessful in procuring any Onchidia on the plants themselves. As a rule, 

 these animals live, like Limaces, in damp places, generally close 

 to tanks or ditches, especially those which are supplied during high 

 tide with brackish water. They also seem to be common on the 

 sea-shore, preferring the damp insular climate to that of large con- 

 tinents. Sometimes they are found in places which come under the 

 influence of high tides. They either crawl about on the high ground 

 between the vegetation, or on old wood and stones, etc. During the 

 rainy season, they are naturally most numerous. When kept in a 

 vessel with water, they often go voluntarily into it and remain for 

 some time there, (as I have observed in Onchidium tiyrinum and 

 pallidum) until they are obliged to appear on the surface for the sake 

 of breathing. In this point they fully agree with the species of 

 Scarabus, and other estuary shells. Onchidium tigrinum sometimes 

 voluntarily remained for 24 hours in brackish water, a small air- 

 bubble being visible near its pulmonary orifice ; Onchidium typha 



