iNCisuEA (scissurella) lytteltonensis. 9 



seen still more clearly by inspection of fig. T , in which the 

 relations of the gill-filaments to the axis are obvious. In the 

 anterior gill-filaments of the left ctenidium the skeletal bars 

 appear to be ventral in position, but this is because these 

 filaments are reflected backwards and their natural surfaces 

 are reversed. Incisura, then, agrees withPleurotomaria, and 

 also with Trochus {fide F I cure and Gettings) and Fissurella, 

 and differs from Nucula. But it must be observed that 

 Woodward went further than the facts warranted when he 

 asserted that the position of the gill-bars indicated a more 

 remote affinity between Pleurotomaria and the primitive 

 Lamellibranchia than is generally supposed. As a matter of 

 fact the skeletal bars differ considerably in position in some 

 not remotely related mollusca. In Solenomya, for instance, 

 they are shifted to a more dorsal position than in Nucula, and 

 in the Filibranchia they are actually dorsal. The fact is, as 

 Woodward himself pointed out, these skeletal bars have a 

 physiological rather than a morphological significance, and 

 are always developed in close relation to the tracts of cells 

 bearing specially long or functionally important cilia. Hence, 

 in Filibranchia we find them related to the ciliated discs, 

 which are near the dorsal edges of the filaments. 



In so small an object as Incisui-a it is very difficult to make 

 sui'e of the presence or absence of a septum dividing the 

 blood-channel into an afferent and an efferent moiety, but I 

 am tolerably certain that such a septum exists, as shown in 

 fig. 18. But it is not always placed transversely, but may 

 be oblique or even nearly longitudinal. 



The attached portion of the axis of the right ctenidium 

 extends far back in the extreme right-haud corner of the 

 mantle cavity, lying close above the columellar muscle of 

 that side, and gives off some three or four short filaments 

 before reaching the level of the osphradial ganglion. At 

 this spot there is a break in the continuity of the filaments, 

 none being formed in the immediate proximity of the ganglion, 

 but in front of it the ctenidial axis becomes free, and drops 

 vertically down in front of the columellar muscle to hang in 



