Oliver. — Mollusca of the Kermadec Islands. 511 



Cellana H. Adams. 



Extremely abundant on rocks between tide-marks throughout the 

 Kermadec Islands, the members of this genus constitute a highly variable 

 and puzzling array of forms, diverging into four distinct types, which I have 

 ranked as species. But though typical examples of these four species cannot 

 be confused, and are distinguished by characters of form and sculpture 

 which undoubtedly would be counted as of good specific value in any genus, 

 yet intermediate forms appear so frequently and of such non-characteristic 

 features, and any form merges by so complete a series into each of the others, 

 that it is impossible exactly to define the limits of the species here accepted. 

 But the differences between the extreme forms are so great that it is in- 

 conceivable that any one could produce any of the others; consequently I 

 have no hesitation in ranking them as species, notwithstanding the multi- 

 tude of intermediate variations. Especially bewildering to the despairing 

 systematist are young shells, where nondescript forms inseparable into well- 

 defined groups gradually pass into all four species. This is a suggestive 

 and perhaps significant fact, for it may be that there are four or more 

 species now in course of production in the Kermadec Group, and the form 

 with the least distinctive characters — that which agrees with the type of 

 C. craticulatus — possibly approaches most closely to the supposed ancestor. 

 A relationship between variation on the one hand and habitat and distri- 

 bution on the other is also indicated,- for, although all the forms were found 

 on Sunday Island, two only — 0. craticulatus prolixus and G. hedleyi — were 

 there dominant, and these showed a tendency to group themselves in 

 distinct belts, prolixus usually occurring higher up on the rocks than 

 hedleyi. C. vulcanicus was chiefly found on the outlying Meyer Island : 

 C. hedleyi corrugata was the only form found on Macauley Island : while 

 C. scopulinus alone was collected on French Rock. 



To sum up, the examination of ample material leads me to the con- 

 clusion that in the Kermadec Group there exist about four species of 

 Cellana in process of being formed out of a single species, and the young of 

 all are frequently so. much alike that a satisfactory disposition is scarcely 

 possible. I can suggest no explanation of any value as to the cause of 

 this great variability. An inherent tendency to vary which must be postu- 

 lated for all organisms apparently has had full scope for the production of 

 variants, while, perhaps through the non-arrival of other competitors in the 

 area, selection has not acted on the variable characters. Cellana is not 

 the only genus in which apparent unlimited variability has taken place 

 in the Kermadec Islands. The case of Pterodroma neylecta is perhaps one 

 of the most remarkable among birds. In this species every variation of 

 colour between a uniformly black bird and one all white except the wings 

 and back may be seen during the breeding season at Sunday Island. 



Cellana craticulatus (Suter). 



Helcioniscus craticulatus Suter, Pro. Mai. Soc. vi. 352, 1905. 

 Recorded, Suter, I.e. 



Shell depressed, height 0-24 to 0-32 of length, broadly elliptical, nar- 

 rowed in front. Apex about one-third the length from the anterior end. 

 Anterior slope nearly straight, posterior slope arched. Margin finely crenu- 

 late. Sculpture : Exterior with numerous (about 70) close, radiating, small, 

 slightly irregular riblets, made slightly uneven or minutely beaded by con- 

 centric growth-lines. Colour : Above dark green, variously marked with 



