326 Mr. J . Alder on new species of Rissoa and Odostomia. 



r Animal yellowish white, with three longitudinal bands of rich 

 brown on the body, and blotches of the same colour on the spiral 

 portion. Tentacula four, of moderate length ; the superior pair 

 rather longer and more slender than the lower. Eyes placed very 

 far behind on the back, large and prominent. Foot produced and 

 strongly notched in front, grooved down the centre. 



Found on small sea-weeds in pools among the rocks at Dalkey 

 Island near Dublin, and at Cullercoats, Northumberland. 



The coloured markings of the animal seen through the shell 

 when ahve have the appearance of belonging to the latter, which 

 is however perfectly colourless, and so transparent, that though 

 the eyes of the animal are placed beneath it, the power of vision 

 is scarcely interrupted. 



The striking peculiarities of this animal are the two pairs of 

 tentacula and the posterior position of the eyes, in which cha- 

 racters it differs not only from Rissoa but from all the allied ge- 

 nera. The lateral appendages of the foot are also absent in this 

 species. These differences are undoubtedly sufficient to consti- 

 tute a new genus, which I shall content myself with merely in- 

 dicating at present. The shell has no character to distinguish it 

 from that of Rissoa, and affords another instance of the difficulty 

 of determining a genus from the shell alone. 



In a notice of this curious little animal communicated to the 

 Natural-History Section of the British Association Meeting at 

 Cork, I described it under the name of R. albella. Since then 

 however I have found a shell, described by Capt. Brown in his 

 ' Conchological Illustrations of Great Britain,^ which appears to 

 I be identical with my species, and have therefore not hesitated to 

 j adopt his name. 



Odostomia nitida. Plate VIII. fig. 5. 



Shell ovate-conical, smooth, shining, transparent, white, of five 

 much-rounded, well-defined whorls, the last occupjdng about two- 

 thirds of the length of the shell. Aperture ovate, entire. Outer 

 lip thin. Pillar-lip scarcely reflected, having an umbilicus behind 

 it, and a prominent tooth a little above the centre of the inner 

 margin. Length rather more than one-tenth of an inch ; breadth 

 about half its length. 



A single specimen from shell-sand at Tynemouth. 



This species has very much the contour of a Rissoa, and were 

 it not for the distinct tooth, it might very readily be taken for the 

 species last described ; it is however larger, a little more elon- 

 gated, and the whorls, especially the penultimate, are more 

 founded. The animal is unknown. 



Dr. Fleming makes it one of the characters of his genus Odo- 

 stomia to have the " peristome incomplete retrally •/' but some of 

 the species he has included in it, as well as the one here described. 



/ 



