Dt HcTclman on the " Olfactory Tubercle.'' 263 



coiled, and having a wide opening between them ; frequently 

 in this form one of the sides is longer than the other (Fig. 10). 



The first complication is produced by the turning-in of the 

 extremities of the horns. This may take place in one or in 

 both, but has not been noticed to any great extent. 



A variety which occurred several times is produced from 

 the simple semicircular form by a thickening at the right and 

 left extremities of the posterior part or middle of the curve, 

 resulting in two short projections like rudiments of a pair 

 of posteriorly directed horns. 



Another variety, which was only noticed once, is a very 

 remarkable form (Fig. 11). It resembles the last one in 

 having the posterior projections, but here they are rather 

 more developed. The anterior horns are somewhat irregular, 

 the right one is bent slightly outwards and the left one in- 

 wards. The great peculiarity, however, is the presence of a 

 third anterior horn, nearly equalling the other two in size, 

 and springing from near the middle of the posterior part 

 of the organ. This is the only olfactory tubercle I have 

 noticed in which such a process running up the centre of 

 the organ was present. 



The characteristic form in this species is undoubtedly the 

 simple curve, and may be described as follows : 



Olfactory tubercle small, simple, semicircular or crescentic. 

 Posterior end rounded, sides symmetrical. Aperture 

 anterior, wide. 



ClONA INTESTINALIS, Linn. 



In this species the olfactory tubercle is small, and is pretty 

 constant in shape. The peri-tubercular area is small, and in 

 many cases can hardly be said to be present ; the tubercle 

 therefore lies in the zona praebranchialis, but does not extend 

 nearly up to the base of the tentacles. 



The usual form of tubercle (Fig. 12) has its lateral extent 

 rather greater than its antero-posterior, is perfectly symmetri- 

 cal, and has both horns turned inwards. The simplest modi- 

 fication is when, without any change in the plan, the tubercle 

 is rendered unsymmetrical by the greater development of one 



