258 ProceedtTu/s of the Boyal Physical Society. 



tubercle, and is not coiled. The left horn, on the other 

 hand, is very long; it first curves inwards towards the 

 right side, then turns anteriorly, and finally sweeps outwards 

 to the left, the resulting form being an S reversed. 



It is extremely difficult in this species to say which 

 variety is characteristic, and in fact it is not possible to 

 take any one form as typical in all its details. The de- 

 scription should leave a margin for variation, and the 

 following is sufficiently general to admit of all ordinary 

 individual variations, and at the same time characteristic 

 enough to be of use in identifying the species : 



Olfactory tubercle large ; elliptical or ovate in general 

 form. Posterior end broad and rounded, sides equally 

 curved. Aperture anterior, central, not wide. Horns 

 of moderate length, curved either outwards or in- 

 wards. 



AsciDiA ASPERSA, 0. F. Muller. 



The olfactory tubercle in this species, though of consider- 

 able size, is smaller than that of Asciclia virginea. The peri- 

 tubercular area is generally symmetrical, and contains about 

 one-half of the tubercle. The zona preebranchialis is narrow, 

 and the anterior end of the tubercle comes within one- 

 third or one-half its height of the base of the tentacular 

 circlet. 



The form of the tubercle is much more constant, and the 

 variations are fewer, than in the case of the last species. In 

 four varieties noticed the shape was symmetrical, the opening 

 being anterior and central, and the horns being equal and simi- 

 larly coiled. The simplest form (Fig. 4) is one in which the 

 tubercle was of an ovate shape, the posterior end being broad 

 and rounded, while the horns anteriorly were bent inwards 

 slightly, so as to approach each other, but without meeting. 

 In the next variety the tubercle was transversely elhptical, 

 and the horns, bent inwards as in the last, were coiled so as 

 to describe three-fourths of a circle, leaving a wide opening 

 between them. Another variety differed only in having the 

 broad posterior end slightly pushed up in its centre, so as to 

 form a re-entering angle, and in the horns being less coiled, 



