NUCULA. 217 



acute. The inside is nacreous, with usually a bluish cast ; 

 the hinge-margin is broad, and provided with about twenty 

 teeth behind and about ten in front, which appear to vary 

 very greatly in elevation, but are typically very long, and 

 most elevated towards the extremities of each series. 



An ordinary-sized individual measured two-fifths of an 

 inch in length, and rather more than a third of an inch in 

 breadth. 



The animal is shaped like the shell ; and is of a yellowish 

 or brownish-white hue. The mantle is open throughout in 

 front and posteriorly, not forming any siphons, and having 

 its margins quite plain. The foot is white, and as if pedun- 

 culated and deeply grooved, so as to expand into a broad 

 leaf-shaped disk with serrated margins : by means of this 

 organ it can creep like a gasteropod, and we have seen it 

 walk up the sides of a glass of sea-water in which we had 

 it confined for observation. The structure of the branchicc 

 and margins of the mouth have been carefully examined by 

 Mr. Clark, whose description we prefer : " The branchiae 

 are pale reddish-brown equal rather elongated lamina, two 

 on each side, running pretty straight longitudinally and 

 hanging parallel to each other, free, plain on their outer 

 surfaces, and finely pectinated on the inner. There are 

 two labia on each side hanging from the anterior end of the 

 branchiae, one being very small, like a minute leaf, of a deep 

 brown colour ; the other white, folded together, with the 

 edges scalloped, puckered, or plaited, forming a furrow with 

 its open part anterior, and terminating posteriorly in a spiral 

 curl, having the extreme point a little cloven ; this labium 

 is three or four times the size of the smaller. Besides the 

 pair of labia in each side there is a long membrane pucker- 

 ed, folded together, forming a furrow (like the larger labia) 

 on each side the mouth, and running downwards, partially 



VOL. II. F F 



