184 Mr. W. Clark on Ceeeum trachea and C. glabrum. 
six or seven minute narrow terraces, one above the other,—you 
may then figure to yourself the form of that in Cecum glabrum. 
This creature marches, and in its course performs exactly the 
same manceuvres as the larger species. I thought the Caecum 
trachea very active, but it 1s far surpassed by this animal ; I put 
one of each in a watch-glass of sea-water, and with a camel’s-hair 
brush gave them a fair start, but the little one beat its compe- 
titor hollow, and accomplished a space of 2 inches in 55 seconds ; 
thus affording a proof, even in the Mollusca, that nature eompen- 
sates for the small volume of the minute beings in giving them 
greater energy, vivacity and quickness. This creature I found by 
admeasurement to be j',th of an inch long, and ;3,th of an inch: 
in diameter. 
I have been thus particular, as it will fall to the lot of very few 
malacologists to see this curious species alive. 
I beg to mention that last week, in the offing at Exmouth, 
six miles from the shore, I dredged in the coralline zone a very 
small specimen of that rare species the Lucina orbicularis of the 
British Mollusca, and Venus orbiculata of Montagu, testante 
Laskeyo, whose figure is an exceeding faithful representation of 
its shape. This species has been considered of very doubtful Bri- 
tish origin, and we feel pleasure in adding it to our Indigena. 
The general aspect of the shell is of the palest bluish white on 
the outside and the same within, with a tinge of yellow at the 
posterior extremity on both sides. 
The longitudinal striz radiate from the very acute beaks to 
the ventral margin, and are most evident at the sides of the shell ; 
these are crossed by the strie of growth, giving it an irregular 
cancellated appearance ; the margin is quite plain and acute. 
There are in the left valve three primary teeth, and a conti- 
guous anterior lateral one, and in the right valve, three primary 
teeth with a receptacle for the lateral tooth of the left, and in 
each valve there are faint traces of posterior lamine ; this is the 
hinge of the genus Circe of the ‘British Mollusca,’ but the 
twenty-four radiating longitudinal flattish costelle eminently 
distinguish this species from Circe minima: I am inclined to 
think Montagu’s shell should be styled Circe instead of Lucina 
orbiculata; that excellent conchologist mentions only two pri- 
mary teeth in each valve, but the third might easily escape de- 
tection, as when he wrote imperfect instruments were in use, 
and perhaps there was a less critical examination of objects than 
in the present day. If this shell is not the Venus orbiculata of 
Montagu, as we confidently think, it must be considered a new 
species of Circe. The minute specimen is brilliantly fresh. 
1 am, Gentlemen, your most obedient servant, 
Wii1aM CLark. 
