Vol. 55.] DE. A. W. KOWE ON THE GENUS MICEASTEE. 531 



The dominant form — that with the upper surface in the shape of 

 a flat arch — is so abundant in the zones of Holaster planus, Micraster 

 cor-testudinarium, and in the base of M. cor-anguinum, that it out- 

 numbers the other species and varieties by at least four to one. Its 

 profile, which, it is interesting to note, is the same as that of 

 M. cor-testudinarium, Goldfuss, is shown in PI. XXXV, lines ii 

 & iv, ISTo. 2. It is of interest to note that the same type can be 

 traced even in the high-zonal series, in the upper part of the M. cor- 

 anguinum zone, though of course the special features are different ; 

 and it may at this stage be pointed out that the broad forms, as 

 shown in PI. XXXY, lines iii, v, & vii, JNo. 2, all show much the 

 same outline. Mere shape is of small value as a guide 

 to species. 



It has become necessary to name and define this group, because 

 it has been the custom of some English palaeontologists to merge it 

 with the 21. cor-testudinarium of Goldfass, tor the reason that it 

 has all the characters of that well-known variety, breadth alone 

 excepted ; and because the latter is merely the broad variety of 

 M. prcecursor in whatever zone it may chance to occur. 



The dominant type, together with its associated variations, is 

 named by me 3f. prcecursor, because it is, in actual fact, the low-zonal 

 ancestor of the true M. cor-anguinum auctorum, and because an 

 unbroken continuity can be traced from the varied forms of this 

 group to equally varied forms in the group of M. cor-anguinum. 

 It has not been easy to find a name for this group, which would at 

 the same time be short and explanatory of the fact that it is from 

 this series of forms that M. cor-anguinum springs. Nothing could 

 be more perfect than the chain of evidence, based on the essential 

 features of the test, which takes us up from the comparatively 

 primitive forms of the Holaster planus-zone to the advanced forms 

 at the top of the M. cor-anguinum-zouQ : the continuity of evolution 

 is unbroken, and not a link is wanting. 



Variations. — The dominant type is exemplified in the flat- 

 arched forma planidorsata shown in PI. XXXV, lines ii & iv, 

 No. 2; but the group includes the very depressed forma Hor- 

 mannice,^ No. 1, of the same lines, the round-arched forma 

 heonensis,^ No. 3, the forma gihbosa, No. 5, the Cayeusi-foim,^ the 

 Bucailli-iovm., and others too numerous to mention. 



Several other shapes could be added, but it is felt that undue 

 multiplication of mere pro file- variations would obscure the issue. 

 One important form has been omitted, the forma carinata, which is 

 seen in No. 4 of lines ii & iv (PI. XXXV), and this is noteworthy, 

 as it foreshadows the strong carination of the high-zonal series. 



Some of these variations have a vertical posterior truncation, 

 others slope anteriorly, like Bucaille's figure of M. Normannice ; and 

 a few others, again, have a moderate posterior slope, after the 

 manner of M. Michetini, Agassiz. Save in the case of true examples 



*■ Bucaille, Bull. See. geol. Norm. vol. viii (1883) p. 29 & pi. vi. 



2 Gauthier, Bull. Soc. Sci. Nat. de I'Yonne, vol. xli (1887) p. 373, pis. v & vi. 



' Parent, Ann. Soc. geol. Nord, vol. xx (^1892) pis. i & ii. 



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