266 Proceedings of the Royal Physical Society. 



anterior part of the tubercle and forcing the aperture down to 

 the posterior end. 



Kupffer, in his description of this species, states that the 

 olfactory tubercle is "ringsum geschlossen." He has evi- 

 dently met with the first variation which is here described. 

 This, however, I do not consider as the typical form, and I 

 give the following description as being that of the tubercle 

 most commonly met with, though I think it very doubtful 

 whether it should be considered in this case as part of the 

 specific description : 



Olfactory tuhcrcle small, unsymmetrical, more or less tri- 

 angular. Posterior end pointed, sides unequal. Aper- 

 ture moderately wide, rather to the left side of the 

 anterior end. Horns unequal, both turned inwards, 

 the right usually much the larger and coiled spirally. 



MOLGULA SP.* 



This species shows a marked difference from all the previous 

 ones in the relative sizes of the olfactory tubercle and the 

 peri-tubercular area. The zona prsebranchialis is narrow, but 

 its dorsal offshoot, the peri-tubercular area, is very large. It is 

 triangular and nearly symmetrical; the small olfactory tubercle 

 is placed nearly at its posterior end, in the angle at the 

 anterior extremity of the dorsal lamina (see Fig. 16). 



The tubercle is very small, simple, and almost constant in 

 shape, but varies in the direction in which it is turned. 



Fig. 16. 



Sometimes it is symmetrical and oval, the posterior end 

 being rounded but rather narrow, the aperture wide and placed 

 at the anterior end, and the horns slightly developed and 

 turned towards each other. 



* Probably an undescribed form. 



