184 J. E. S. MOORE. 



large numbers of these shells on the spot^ and to have made 

 collections of the extreme and intermediate varieties, it is hardly 

 conceivable that he would have ever regarded their variations 

 as specifically distinct. Be this as it may, however, it is quite 

 evident, when large numbers of these shells are studied, that 

 their varieties cannot be regarded as specific forms. The shells 

 ofTyphobia Horei are, however, undoubtedly polymorphic, 

 for there are about four well-marked varieties, into one or 

 another of which the great majority of the specimens I collected 

 tend to fall (figs. 13, 16, 23, 25). When the shells are very 

 young, about the time of birth (figs. 27, 28), they are desti- 

 tute of all but the merest trace of spines as well as of the 

 pronounced rostral beak, which is a marked structural charac- 

 teristic of the majority, but by no means of all the older 

 shells. Now the fact that in some adult shells the rostral 

 beak is entirely wanting (figs. 16 and 20) shows that, in 

 this respect, such shells have not deviated from their embryo- 

 logical character. All the four extreme types of variation 

 graduate off into forms which, in the more or less complete 

 absence of a rostral beak, approximate to the shells repre- 

 sented in figs. 16 and 25. Such shells, therefore, may be said 

 to represent the extreme of least specialisation. The remaining 

 extreme polymorphs (figs. 13, 23) can be all traced through 

 successive stages from the types represented in figs. 12, 17, 

 18, and 21. Thus between the extremes, figs. 13 and 16, 

 there are intermediate forms, such as figs. 12 and 17; the 

 extreme, fig. 23, has intermediates, such as figs. 18 and 21, 

 while the extreme, fig. 25, is connected up by forms similar 

 to that represented in fig. 20 and perhaps 21. Any number 

 of intermediate stages could have been represented for each 

 series ; but for obvious reasons those only have been selected 

 which seemed to best express the transition from any one type 

 to the next. It would thus appear that Bourguignat's four 

 species were formed in the absence of an adequate quantity ot 

 material to work upon, and this conclusion is finally clinched 

 by the fact that the anatomical characters of the soft parts of 

 the extreme variations are indistinguishable from one another. 



