264 Proceedings of the Royal Physical Society. 



of the horns, or by one side of the organ being more raised 

 than the other. 



Occasionally a variety is found in which one of the horns 

 is turned outwards, so that both are coiled in the same 

 direction. Heller, in his " Untersuchungen ueber die Tuni- 

 caten des Adriatischen Meeres," mentions having met with 

 this form. 



Fig. 12. Fig. 13. 



A third variation, which only occurred in one specimen, 

 is caused by both horns being turned outwards, one to the 

 right, the other to the left (Fig. 13). The former is very little 

 developed, having merely a slight hook at its end, but the 

 latter is of considerable length, and sweeps round in a 

 large curve, forming nearly a complete circle. The appear- 

 ance of this variety is peculiar, and very unlike the normal 

 form. 



In this species the variations occur very rarely, conse- 

 quently the following description of the typical form may be 

 used for purposes of specification : 



Olfactory tubercle small, symmetrical, broadly cordate. Pos- 

 terior end broad, and rounded ; sides equally curved. 

 Aperture anterior, moderately wide. Horns equal, 

 both turned inwards. 



Styela grossularia. Van Beneden. 



In this species the peri-tubercular area is comparatively 

 large, and encloses the whole of the olfactory tubercle. The 

 zona praebranchialis is narrow, and the tubercle is only 

 separated from the base of the tentacles by a space equal to 

 its own length. 



The tubercle is small and very variable in form. There 

 are two marked types, each of which has several varieties. 

 The first has no aperture ; the tubercle varies in shape from 

 nearly circular through various ellipses to a very elongated 



