Mr. W. Clark on the genus Lepton. 131 



has a linguiform tapering aspect, but this part almost imme- 

 diately, after feeling about, ranges itself anteriorly and hori- 

 zontally ; and at the same time, on the other side of the pedicle, 

 a bevelled, attenuated, pointed portion issues, somewhat shorter 

 than the first ; this is longitudinally cloven as far as the pedicle, 

 and can form a sort of oval disk, but on the march it is rarely 

 expanded : at the base of the cleft is the byssal gland, which oc- 

 casionally pours out a glutinous red filamentous matter, that in 

 confinement is copious, and discharged anteriorly, which at first 

 I thought was faecal matters, and was puzzled to account for such 

 an issue anteally, but the subsequent view of the single sessile 

 posteal anal conduit and the ejection of pellets cleared up the 

 difficulty. This foot is in every respect similar in miniature to 

 that of the Pedunculus pilosus and of the Arcadce. 



The animal is vivacious, and allowed itself to be examined 

 many times daily ; it marched with quickness, but I only once 

 saw it progressing in a vertical position ; the usual posture of the 

 shell is to rest on one of the disks, which is frequently changed 

 for the other ; the adductors did not appear to allow of a greater 

 opening of the valves than the ordinary extent. The animal, when 

 placed at the bottom of a glass, always crawled up and moored 

 itself by a filament at the side ; sometimes, however, it slipped its 

 moorings and floated free on the surface of the water with the 

 umbones downwards, and after an interval refixed itself by spin- 

 ning a byssal thread. 



I cannot speak at present of the branchiae and palpi, as the 

 animal and shell are in my collection, and are thus preserved to 

 show that the shell, though usually described by conchologists as 

 gaping, can, in consequence of the flexibility of the thin laminar 

 valves, be completely closed. There is no branchial siphon ; but 

 there are mantellar folds, which, with the great ventral opening, 

 amply provide for the admission of the water. 



The animals of this interesting group exhibit, in the tentacular 

 filaments and curious foot, as well as in the sculpture of their 

 shells, very considerable variation from Kellia rubra and Kellia 

 sub orbicularis ; the types of one of the genera of the family in 

 which they have been located by authors, doubtless from the 

 want of knowledge of the animal. Taking into consideration 

 that the Leptons have many of the attributes of the Arcada, and 

 especially giving due weight to the remarkable similarity be- 

 tween the foot of Lepton squamosum and the Pectuncnlus pilosus, 

 I am almost induced to believe that it is in a false position, in 

 connection with the Kelliada, and that it ought to follow or pre- 

 cede Galeomma, which, with me, is an undoubted genus of the 

 Arcada. 



The punctures of this species and of L. conveomm are in the 



9* 



