DORSAL TUBERCLE OF ASCIDIA. 99 
. The other horn sometimes becomes longer, (No. 15) 
and turns outwards, resulting in the form of tubercle 
seen in No. 19. | 
. Or else it gradually curves over in same direction as 
the first horn (Nos. 16 and 17) which results in the 
form shown in No. 18. 
. Where both horns turn inwardly, the resulting shape 
is shown in No. 20. 
. An unusual variety of the ovate tubercle occurs when 
one horn bends sharply inwards, and then continues 
its course anteriorly. (No. 22). 
. Returning to the simple, circular form of tubercle, the 
first variety occurs when one horn curves inwards 
and turns posteriorly. (No. 27). 
. Both horns may turn slightly mwards. (No. 28). 
In this specimen the posterior end, to the right of the 
middle line, was flattened and bore a slight projection 
on the inner side. 
. Both horns turn in more decidedly and the aperture is 
very small. (No. 29). 
. Two specimens showed a prolongation anteriorly from 
the right horn after it had turned up posteriorly. 
(No. 30, 31). 
. The extreme ends of the horns of the simple circular 
tubercle sometimes run anteriorly and parallel, with 
a varying distance between them. (Nos. 32, 33). 
. These parallel ends, then turn outwardly but do not 
bend posteriorly. (Nos. 34, 35). 
. When these ends curve posteriorly, the result is shown 
in No. 36. This specimen had a slight projection 
inwards from the posterior end of the tubercle. 
. Both horns bending in the same direction produce the 
form shown in No. 87. 
