^^^' 55*] ^^' ^' ^' ROWE Olf THE GENUS MICRASTEE. 525' 



(2) Group of Micraster Leskei. 



M. Leskei, Desm., 1837. 



The figures given by the following authors are accepted as being' 

 good representations of the general shape and appearance of the test,, 

 but for the essential details, which, after all, are the only real guide 

 to a species, the reader is referred to PL XXXYIII, figs. 1, 2 & 3, 

 and to PL XXXVI, fig. 2 :— 



CoTTEAu, ' Echinides fossiles de I'Yonne,' pi. Ixxvi, figs. 2 & 3. 

 Cayeux, 'Mem. sui' la Oraie grise du Nord de la France,' Ann. Soc. geoL 

 Nord, voL xvii (1889-90) p. 105 & pi. iii. 



Weight, ' Brit. Cret. Echinod.' Monogr. Pal. Soc. pi. Ixii a, figs. Sa & b. 



The name of M. Leskei has been retained, because that of M. brevi- 

 jporus, Ag., 1840, is a nomen nudum, and because the Agassiz cast 

 (Centuria M, 10) in the British Museum (Natural History), Crom- 

 well Road, in no way conforms to what we understand by this 

 species, being apparently merely a young example of M. cor- 

 anguinum. 



JFor a complete synonymy up to 1857 the reader is referred to 

 Cotteau's * Echinides fossiles de LYonne,' vol. ii, p. 352, and for 

 more recent writings to those of L. Cayeux^ and Y. Gauthier,^ the 

 latter of whom has written a very thoughtful paper on the affinities 

 and varieties of this species, clearly setting forth the difficulty of 

 knowing where to draw the line, in separating the small from the 

 large forms, and the large examples from other allied low-zonal types.- 

 The lack of familiarity on the Continent with M. cor-bovis has still 

 further increased the confusion. 



While nobody, who is in any way familiar with the species under 

 discussion, would have any difficulty in determining the identity of 

 a small and typical example, it is equally clear that, as soon as 

 we pass from the small and typical forms, we meet with a 

 number of larger and closely -related forms, which we have 

 great difficulty in referring to the type. There is probably no 

 species whose exact limitation is fraught with so many difficulties ;. 

 but it is hoped that, by means of the photomicrographs which illus- 

 trate this paper, it can be demonstrated that we have means at our 

 disposal for defining the species within more rigid limits, and, by 

 tracing the passage-forms, to indicate at what point we must draw 

 the line. It will not be necessary to go minutely into all the 

 features of the test, as we have done in the case of M. cor-bovis,. 

 but attention will be drawn to certain essential characters, which 

 will enable us to separate this species from its passage-forms; for,, 

 once having eliminated M. cor-bovis from the discussion, we have 

 only the passage-forms to deal with. 



Now, in England, at any rate, we look upon M, LesJcei as a small 



"^ Op. supra cit. 



^ ' Descr. des Especes de la Craie de Eeims,' Bull. Soc. Sci, Nat. de iTonner 

 Tol. xli (1887) p. 8. 



