116 Mr. W. Clark on Assiminia Grayana and Rissoa anatina. 



great protrusion of the neck and rostrum ; the latter organ in 

 every position is always borne much in advance of the foot ; it is 

 vertically cloven, and at the under part, in the centre, slightly 

 so, in a crosial direction, from whence the jaws or subcircular 

 arches of the lingual riband, supported by the buccal plates, are 

 almost momentarily exserted ; the oesophagean portion of the 

 riband springing from the stomach is short and of rissoid stamp. 

 The posterior part of the muzzle next the neck is suffused in 

 different individuals with all the phases of dark colour ap- 

 proaching to almost black, and with all the variations of the 

 most delicate cinereous hues ; the flattened or expanded anterior 

 curved terminal portion abruptly becomes white, and is shot 

 with the minutest points or flakes of a still intenser white. A not 

 very deep longitudinal groove, or two contiguous parallel lines, 

 are visible on the neck when moderately protruded, but these, 

 when greatly so, are lost ; they are generally supposed to act as 

 a conduit to the branchial water. 



The dark tentacula spring from a minute mammilla, and 

 are very short, massive, and columnar at the base, becoming, 

 though still thick, somewhat spatulate and rounded at their 

 whitish terminations ; in the centre of this minute plateau, 

 at some little distance from the extremities, the large eyes 

 are immersed, and from the intensity of the black colour are 

 almost equally visible on the upper and lower surface ; this is 

 their natural position when the animal is on the undisturbed 

 march, but the instant it is disquieted, both the tentacula and 

 eyes assume various modifications of figure : for instance, when 

 the neck is greatly exserted, or the animal is in comparative re- 

 pose, in either of these opposite conditions they become much 

 contracted, especially at the tips, which are folded or withdrawn 

 into a minute hollow, out of which the eyes peep, and thus ap- 

 pear fixed at the extremities ; but, as we have shown, this is not 

 their true site, which here, and in all the Truncatellcs, is at some 

 little, but distinct, distance from their final points. 



Some authors have thought that the eyes are mounted on 

 pedicles, connate, and of the same length as the true tentacula : 

 this idea is wrong : we repeat their true position is an absolute 

 immersion in the tissue ; the tentacula are therefore strict vibra- 

 cula, and in nowise sustentacula. The white pupil mentioned 

 in my first account of Truncatella was not detected in this spe- 

 cies. The animal on the march carries the tentacula nearly at a 

 rectangular divergence : this position is a marked characteristic 

 of the genus Truncatella, 



Foot rather short, broad and fleshy, not auricled, but subqua- 

 drate in front, with a tendency to roundness at the external 

 angles, without a central sinuation, and deeply and conspicu- 



