438 M. J. Brock on the 



bvit the great majority of them form a continuous series 

 between the larger warts and the mantle-margin, and show 

 exactly the same irregularity of arrangement as the larger 

 warts themselves. At the first glance the two kinds of struc- 

 tures seem to have little to do with each other ; frequently 

 the smaller ones appear as mere scar-like shrinkings-in of the 

 surface of the mantle without any perceptible elevation above 

 the surface. But more advanced structures, in which these 

 scar-like shrinkings already surround a slightly convex 

 eminence, show us the transition towards typically con- 

 structed warts. These smaller structures are warts in course 

 of development ; the only thing remarkable in the process 

 of development is the circumstance that the semicircular 

 furroAV which girdles the wart ventrally first sinks into the 

 surface of the mantle, and only then the wart rises up above 

 the inner mantle-surface from the dorsal declivity of this 

 " fosse." This idea is also fully confirmed by the examina- 

 tion of series of transverse sections. 



In general the smaller structures just described are so little 

 remarkable in the vicinity of the mantle-margin, even in 

 chromic-acid preparations, that during life the slight differ- 

 ences of relief cavised by them will hardly appear at all. On 

 the other hand, it would appear from Vaillant's description 

 that during life these structures also are the seat of an intense 

 pigmentation, and therefore must catch the eye very readily. 

 With regard to them our author says {I. c. p. 83) : — " In the 

 periphery of the free margin of the mantle the green colora- 

 tion forms an uninterrupted border, which is adorned with a 

 series of very regularly arranged black spots ; near them, but 

 further inwards, there are large projecting tubercles also 

 marked with a black spot ; tliey are more numerous in the 

 vicinity of the branchial aperture, and represent the eye- 

 tentacles." 



While there can be no doubt as to the identity of these 

 latter structures (which, moreover, are described more in de- 

 tail in another place, I. c. p. 135) with the larger warts described 

 by me, the agreement of the younger structures with Vail- 

 lant's series of pigment-spots along the mantle-margin is a 

 good deal more uncertain. In favour of this view we may 

 cite the similarity of situation and arrangement, although so 

 regular a position as Vaillant describes and figures {I. c. pi. viii. 

 fig. 1) does not agree with my objects, and, further, the cir- 

 cumstance that the larger warts are most certainly pigmented. 

 On this point, unfortunately, my personal recollections leave 

 me completely in the lurch. 



In order to ascertain the intimate structure of all these 



