Yol. 55.] DR. A. W. EOWE ON THE GENTJS MICRASTER. 499 



II. A EbVIEW of THE EVOLUTIOJ!^ OF SpECIAL FEATURES OP THE TesT 

 IN ALL PORMS AND AT EACH HoRIZON. 



This analysis will show how completely the conditions of existence 

 in the various zones dominate all the special characters of the test, 

 and thus what have been hitherto considered purely specific cha- 

 racters, may now be regarded from the much broader standpoint of 

 horizoual influence, finding their true level in the scheme of pro- 

 ■^ressive evolution. 



The percentages are given for all the forms collectively, as con- 

 tained in each zone, except in certain cases where a species or 

 variety is particularly specified. To clear the ground, before dealing 

 with the species and varieties of the genus, it may be well to pass in 

 review those features of the test which experience has shown to be 

 useful as guides to horizon ; and at the end of this detailed review 

 will be found a Tabular Summary (pp. 512-513), which will show 

 at a glance the progressive evolution of each character. Only those 

 features of the test with which it is necessary to deal in detail will 

 be mentioned now. 



Interporiferous area. — In 1855, Hebert made observations 

 on the ambulacral plates, and considered that he had found therein 

 a guide to distinguish the different species. His figures appear in a 

 paper on the Meudon Chalk, but there seems to be no text to 

 ■elucidate the plates.^ 



Erom an entirely different standpoint — that of horizon — the 

 present writer also attaches importance to the interporiferous 

 portion of these plates, and, to facilitate reference, figures are given 

 of five distinct types of the interporiferous areas, which, together 

 with the other features of the test, will be found most useful in 

 determining the horizon from which any given example is derived. 



With the exception that the smooth area is found oiily in the 

 zones of Rhynchonella Cuvieri, Tei^ehratulina gracilis, and Holaster 

 planus^ and that it indicates either Micraster cor-hovis or M. Leshei, 

 these areas have no value in the determination of species, as they are 

 really an index of horizon. Their true interest lies in the fact that 

 they assist in marking the passage from one form to another, and 

 that they throw a side-light on the evolution of the different varieties 

 of the genus. 



Eor the purpose of illustration, this interporiferous area is divided 

 into five classes — ' smooth,' ' sutured,' ' inflated,' ' subdivided,' and 

 ' divided' (PI. XXXVI, figs. 1-8). 



The smooth area is restricted in most English sections to 

 M. cor-hovis and M. LesJcei, and 20 per cent, of all forms from the 

 zone of H. planus fall within this category. Practically, all the 

 specimens from the zones of Bh. Cuvieri and T, gracilis are of this 



^ ' Fobs, de la Craie de Meudon,' Mem. Soc. geol. France, ser. 2, vol. v 

 (1855) pi. xxix. 

 ^ ^^ 2k2 



