Mr. W. Clark on the Genus Truncatella. 7 



the slightest tinge of brown on the neck, the effect of that colour 

 in the buccal plates declaring itself through the membranes. 

 Mantle plain and thickened at the margin. Eostrum annulated, 

 but not so long in proportion as in its congener, broad, flat, 

 emarginate at the extremity, forming on the right and left a 

 flattish arcuated lobe ; beneath, vertically and slightly crosially 

 cloven, having the usual corneous buccal plates, tongue, and pair 

 of white jaws. Tentacula very short, broad, flat, rounded at the 

 end, divergent, both in quietude and on the march to almost 

 right angles, with large eyes and dilatable white pupils as in the 

 preceding species, placed not positively at their terminations, 

 but on the centre of the membrane, at some distance from the 

 extreme points : this was particularly remarked ; but the very 

 powerful lens used would give them that appearance, which of 

 course would not be so apparent when viewed by a common 

 glass ; though, nearer the extremity of the tentacula, the cha^ 

 racters are precisely those of T. Montagui, and a similar white 

 pupil is a singular coincidence. The neck, as in its congener, 

 has the same longitudinal groove or canaliferous lines running 

 medially through the rostrum and neck to the branchial leaf, and 

 is doubtless an aqueduct. The neck and rostrum in quietude 

 appear short ; they are so in T. Montagui, but can in like man- 

 ner be greatly extended, though not proportionately so much ; 

 for these reasons the branchial streamlets and nervous ganglions 

 were not seen : but I distinctly observed, through the tenuity of 

 the shell, the kidney-shaped respiratory apparatus with the con- 

 striction at the end nearest to the heart, and cleai-ly detected 

 the fine blood-vessels of the reniform loop ranged in transverse 

 order. The only difference observable in the two species is, that 

 in this, the leaf appears of a shorter or more oval contour, more 

 compact, and with a less deep constriction ; the neck and rostrum 

 have no additions beyond the groove and annulations. Foot 

 thick, having a more elongated oval shape than in its congener, 

 and, as in it, on the march is rounded in front and behind ; 

 it simulates the same transverse crease or line on the sole, 

 and has a like character of progression. There is a simple, 

 almost terminal, but decided operculigerous lobe that bears a 

 light corneous suboval operculum, which in all points, except 

 being of lighter colour and broader oval, is identical with that 

 organ in the preceding species to which I refer ; indeed so great 

 is the similarity of the two animals, that I might by a reference 

 have condensed the greater part of this account ; but as these 

 notes are decidedly comparative of two particular creatures, I 

 have, for impression, given them in extenso. The animal is free, 

 but deliberate on the march, and carries its shell very upright, 

 at near an angle of 75"-80° from the horizon. 



