Tol. 55.] DR. A. AV. EOWE OlST THE GENUS MICEASTEK. 515 



appeals to me as universally applicable ; and therefore a plain milli- 

 metric measurement has been adopted as being more generally useful. 



The length is taken by dropping the specimen between the 

 Yertical arms, and measuring from the most prominent anterior 

 and posterior points. It will thus be seen that the anterior 

 measurement is taken from the anterior inflations, and not from 

 the bottom of the ambital notch ; and that the rostrum, when 

 it exists, is designedly included in the posterior limit. This is done 

 because the rostrum is not a meaningless excrescence, but has 

 discinct value as an index of horizon. 



The breadth is measured at the greatest transverse diameter, 

 and the height at the highest point of the llicraster, independently 

 of the fact that the point in question is probably not the apical disc. 

 Here, again, the position of greatest height is not without its 

 meaning, for in the lower zones it is either at the apex or immediately 

 behind it, and in the higher zones it is much farther back, on account 

 of the development of the carina. 



The position of the apical disc is obtained by placing each end of 

 the specimen in turn squarely against the fixed vertical, and laying 

 a small millimetre-measure across the top of the Micraster. 



The position of the mouth is not taken from a line drawn across 

 the notch from the anterior inflations, but from the bottom of the 

 notch itself, and a glance at an example from the zones of Holaster 

 planus and ^larsiijntes testudinarius will make it clear that the depth 

 of the notch is so different in the two forms that it would yield 

 very misleading results. Practically, all examples from the lower 

 zone have a shallow notch, and all from the higher zone a deep one, 

 and on examining a hundred specimens of Micraster from the 

 Marsu2ntes-zone it was found that, by measuring from an imaginary 

 line across the notch, the average position of the mouth was the same 

 as in the low-zonal forms ; whereas, in reality, the mouth in the 

 iow-zonal forms is invariably distant from the margin, and in the 

 high-zonal series it is as constantly close to the notch. The mouth, 

 then , is measured from the bottom of the notch to the anterior margin 

 of the peristome. The anterior margin of the peristome is chosen 

 because the labrum is often broken, and because its length varies 

 greatly in the higher and lower zones, and is also useful as a guide 

 to horizon. 



AVe may now consider the species and varieties found in the 

 zones treated of in this paper : — 



M. cor-bovis. j 4 §= ^^ ^g^^^.^ j ^ gJ 



{ 2. o M. prcBcursor. 1 § ^ 



' ■ M. coT-te&tudinarium. ) '^ 



"^ 8* »-^ M. prcBCursor (in part). \ 55 



M. :pr<scursor. \t'%l ^- cor-testudinariicm (in part). I | .^ §= 



M. cor-test'udinarium. j §" S" 2j ^' cor-anguinum auctorum. I I i ^ 



! § 



cor-anguinum var. latior. 

 2l2 



