Bt Herdman on the '' Olfactory Tuhercle!' 261 



There is little difficulty in this species in considering as 

 typical the simple form, which may be described as follows : 



Olfactory tubercle rather large, symmetrical, ovate. Pos- 

 terior end rounded, wider than anterior ; sides equally 

 curved. Aperture anterior, moderately wide. Horns 

 equal, sloping slightly inwards. 



ASCIDIA PLEBEIA, Alder. 



Here the peri-tubercular area is almost entirely rudiment- 

 ary. At the utmost it encloses only the extreme posterior 

 end of the tubercle, and in many individuals there is no de- 

 pression at all, so that the tubercle rests on a flat surface, and 

 is, in its entire extent, within the zona pr?ebranchialis. 



"v^ 



Fig. 8, Fig. 9. 



The tubercle is small, and is very variable in form. It is 

 very commonly U or V shaped, with the horns turned in- 

 wards. The posterior end is generally rounded and the sides 

 parallel, one of them very frequently extending further for- 

 ward than the other. In this case the shorter side, which is 

 sometimes the right and sometimes the left, is generally quite 

 straisjht, while the longer one is bent inwards. A varietv oc- 

 curs (Fig. 8) in which the tubercle is rather broad, and both 

 horns are bent sharply inwards and extend down the inside of 

 the tubercle, running parallel with the sides, almost to the 

 posterior end. There is another form in which both horns are 

 turned inwards, but in which the form of the tubercle is very 

 different. It is narrow at the posterior end, and the sides 

 slope outwards ; moreover, the one side is considerably longer 

 than the other, and the horns do not point posteriorly, being 

 coiled. 



In the V-shaped type, which occurred once or twice, the 

 tubercle, leaving the horns out of consideration, was sym- 



