184 Mr. W. Clark on Gsecum 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 Caecum glabrum.- 



This creature marches, and in its course performs exactly the 

 same manoeuvres as the larger species. I thought the Ccecum 

 trachea very active, but it is far surpassed by this animal ; I put 

 one of each in a watch-glass of sea-water, and with a cameFs-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 compen- 

 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 ^th of an inch long, and T ^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 strise 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 strise 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 laminae ; this is the 

 hinge of the genus Circe of the ' British Mollusca/ but the 

 twenty-four radiating longitudinal flattish costellae 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. 

 I am, Gentlemen, your most obedient servant, 



William Clark. 



