Vol. 55.] DK. A. Vf. ROWE ON THE GENUS MICRASTEE. 505 



marred by excessive deposit of carbonate of lime. It may be stated, 

 however, that many photomicrographs have been made of the disc, 

 and that the same progressive ornamentation may be traced as that 

 found in the ambulacra ; still, for the reason given, the disc does 

 not lend itself to rapid examination in the field, and it has been 

 discarded as an unnecessary detail. It may be mentioned, however, 

 that one finds varying degrees of divergence in the posterior genital 

 pores, and that the four pores are sometimes decidedly unsymmetrical. 

 I do not follow StoUey ^ in regarding this asymmetry as a specific or 

 varietal feature, but strongly dissent from such a course. 



Highest point of test. — Here, again, there is a steady pro- 

 gression, which is only what would be expected, as the highest point 

 becomes more posterior in proportion as the carina becomes more 

 developed. 



Anteal sulcus and ambital notch, — All low-zonal forms 

 have a shallow sulcus and notch, and they may be absent in 

 Micraster cor-hovis and M. Leshei. 



This shallowness is an horizonal and not a specific feature ; for 

 it is not more marked in M. cor-testudinarium, Goldfuss, than in 

 Jf. prcBcursor, There is little or no difi'erence in the degree of 

 shallowness in the Holaster planus and M. cor-testucUnarium-zones, 

 but as soon as one passes into the base of the M. cor-anguinum-zoiie 

 the notch and sulcus both deepen, and in the high-zonal forms they 

 are almost invariably deep. The only exception to the last statement 

 will be found in the broad high-zonal varieties, the 31. cGr-anguinum 

 var. latior of this paper, which have a tendency to preserve the 

 shallow notch. Once more we notice a progressive increase of a 

 feature of the test. 



Position of mouth. — Here is another unbroken progression 

 from low-zonal to high-zonal forms, and the table clearly brings 

 •out the average distance from the bottom of the notch. 



Lab rum. — The progressive increase in this feature is also well 

 «een in the Tabular Summary. Its rudimentary condition is observed 

 in M. cor-hovis and in M. LesJcei, and one is even taken a stage farther 

 back in M. Sanctce-Maurce, Gauthier, of the Ehynchonella Cuvieri- 

 zone, where the labrum is practically non-existent. 



It is feebly developed in all the species and passage-forms of the 

 zones of H. planus and M. cor-testudinarium^ and it never covers-in 

 the peristomal orifice until we reach the base of the M. cor- 

 unguinum-zoBe ; but in the high-zonal forms it becomes very thick 

 and prominent, and generally projects well beyond this orifice. 



Labral plate. — In all low-zonal forms the tubercles abruptly 

 stop short of the labral tip, and no secondary tubercles are seen 



^ ' Kreide Schl.-Holsteins/ Mitth. Mineral. Inst. Univ. Kiel, vol. i (1892) 

 p. 191. 



