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his conception of a *' variety," nor attempts any such proof. The 

 remarkable variation of form which obtains in this genus is more 

 than once commented upon ; but we think that its importance was 

 not sufficiently recognised in dealing with species, nor does it seem to 

 have been noted that a large series of specimens from any one of 

 several horizons furnish the most gentle gradation between several 

 so-called species. Several of the species described in the Monograph 

 are clearly but varieties of others. A good example of this is given 

 on plate ii., where fig. 3 does duty as a somewhat rotund example of 

 C robusta, and figs. 7 and 8 as examples of C. ru-gosa. The majority 

 of palaeontologists would unhesitatingly put C. rugosa as a variety of 

 C. robusta, a view held by Salter. The author has failed to find more 

 than two specimens of C rugosa, and neither of them seems to be 

 complete. 



The same failure to distinguish what constitutes a species is seen 

 in the case of the genus Anthvacomya. Fig. 4, plate xiii., represents 

 the type-specimen of A. dolabrata, and is therefore not open to 

 question. Six other figures on the same plate are said to represent 

 specimens of this species, but to us they agree much more closely 

 with A. adamsii. The only other figure upon this plate which could 

 belong to A. dolabrata is fig. 14, and is labelled A. niodiolaris. A 

 comparison of fig. 4, the type, with fig. 5, which should be the 

 same species, will show that the ventral border of the type is almost 

 parallel with the hinge-line, while in fig. 5 it rapidly falls away 

 from it posteriorly. If the hinge-lines and ventral borders of the 

 figures upon plate xiii. be projected forwards with pencil until they 

 meet, the resultant angles of forms said to belong to the same species 

 are very instructive. 



The genus Carbonicola, as now constituted, includes eighteen 

 species, of which five are new. Two species are founded upon only 

 two specimens each, and a somewhat similar occurrence is noted 

 under Anthvacomya, where two species are founded upon one specimen 

 each, in one case an internal cast. Remembering the great variation 

 of form, it is difficult to understand what good purpose can be served 

 by the creation of species such as these. 



The value of the specific forms of Carbonicola is not clearly deter- 

 minable in the Monograph. C. acuta runs off in one direction into 

 C, robusta, in another it first merges into C. subconstricta, and then 

 passes on into C. aquilina ; in still another direction it becomes 

 C. ovalis, which Dr. Hind acknowledges to be little better than a 

 variety. C. obtusa seems identical with C. ovalis. If C. polmontensis 

 be a good species, then figs. 11 and 15 of plate vii. ought to belong 

 to this species, although regarded as C. subconstricta ; but the value of 

 the former is doubtful, and the latter are almost certainly internal 

 casts of C. robusta. The specimens figured 6 and 7, plate ix., are 

 taken as types of C. aquilina, but they might equally well serve as 

 elongated forms of C. acuta, and it is comparatively easy to find a 

 place for them in a series from a single horizon, which would lead up 

 to C. robusta. 



Taking Carbonicola as a whole, it would seem that the genus 

 started with a form of which C. antiqua might well serve as type, and 

 that variation took place along several lines, and we believe upon the 

 same line more than once. It therefore happens that between most 

 of the recognised species there are all sorts of intermediate forms. 

 Not unfrequently in our experience a mass of 50 to 100 specimens 

 collected from one small area of a " mussel band," has yielded at 

 least four species and the necessary intermediate forms. A reference 



