TESTACEA. G9 



The animal Ls thick, with a flat fleshy portion protruding as the foot, 

 folding thinner over the shell, bearing much resemblance, both in aspect 

 and motions, to the tongue of a quadruped. I have not observed it used 

 as an organ for progression. The broader portion of the animal is formed 

 as if into two cavities, the broader beset with external fleshy short 

 processes. 



Animal yellow. Shell darker and lighter shade of yellowish brown. 



This animal is plentiful in Lochend, near Edinburgh. 



Plate XIII. 



Fig. 3. Anodon avonensis (A. cygneus). 



4. Ctclas cornea. — Plate XIV., Figs. 1, 2, 3. 



Shell of somewhat globular appearance : three eighths of an inch in 

 diameter — less in thickness. 



Animal supposed of ascidiau nature ; large in proportion to the shell, 

 extending about twice as much. The exterior consists of two orifices, in 

 low conical prominences ; the opposite portion protrudes as a foot. The 

 animal is white : the shell darkens to horn colour, from age and situation. 

 Young specimens, protected from mud, are transparent and whitish. 

 Some adults also are very transparent. 



The motions of this species illustrate that of the analogous genera. 

 It is to be remembered that the Ascidice, with which we are most familiar, 

 are stationary — necessarily so — from adhesion. But, in the present race, 

 when the foot extends, it adheres and drags the shell towards its ex- 

 tremity, when another step is taken. Thence, the portion with the ori- 

 fices is always behind ; so that motion must be accounted retrograde. 



Specimens have spawned in September, but the spawn, though ad- 

 vancing considerably, did not attain maturity. Several embryos are 

 contained in the deposit. 



It is difficult to obtain a complete and satisfactory view of the crea- 

 ture, which will often continue pertinaciously for a long time within the 



