514 . DE. A. W. KOWE ON THE GENUS MICRASTEE. [Aug. 1 899, 



Take another boundary-line — that between the zones of M. cor- 

 testudinarium and M. cor-anguinum. The shape and general 

 appearance of M. proecursor and M. cor-testudinarium in their several 

 zones are much the same to the casual glance ; but as soon as we enter 

 into the base of the higher zone the ambulacra become shallower, the 

 labrum longer, the tubercles of the labral plate more columnar in 

 arrangement, the plate itself narrower at the plastronal end, the 

 labral tip more granular, and the periplastronal granulation more 

 mammillated. 



So fully has the writer learned to depend on the various features 

 of the test, which have here been brought forward in detail, 

 that when Mr. W. Hill took to Margate some fifty specimens of 

 Micraster, collected by his own hand from definite zones in various 

 parts of England, each specimen was referred to its zone, and in 

 not a few instances a suggestion was made as to the particular 

 part of the zone from which the example was derived. A fossil 

 can hardly be expected to aff'ord more definite information than this. 

 There is ground, therefore, for the assertion that the Micrasters 

 alone will suffice for fixing the junction-line in the low-zonal series 

 of beds. 



Marked as are the differences in detail between the low -zonal 

 and the high-zonal forms, the actual features vary only within 

 certain limits, and the difference consists merely in a progressive 

 elaboration of the epistroma. Let it also be clearly stated that the 

 difference between the various zonal types are not merely distinctions 

 of the study, but that they can be readily and rapidly used in the field. 



When nearing a junction-line, it is the writer's invariable custom 

 to clean the Micrasters every 2 or 3 yards, and a stoat brush 

 and an ivory manicure-instrument are all that is necessary for the 

 purpose. The ivory tool does not damage the test, as a knife would 

 do, and the chalk can be bruised off the test with gentle taps of a 

 light tack-hammer. 



III. Method oe Measurement of the Test, with a Sxnoptical 

 Contrast between High-zonal and Low-zonal General 

 Features oe the Test. 



Before describing the various species and varieties as they are 

 found in the sections mentioned at the beginning of this paper, it 

 may be well to touch on the question of the method of measurement. 

 The more one reads the literature of the genus Micraster, the more 

 one is impressed by the difference in the measurements given, par- 

 ticularly in the position of the apical disc. It has been impossible 

 to find any mention of a fixed method of measurement, and in 

 order that dimensions given in this paper may be capable of being 

 checked by other observers, the plan herein adopted is set forth. 

 At Dr. Gregory's suggestion, a millimetre-scale was used, made with 

 a fixed and a sliding vertical arm, on the principle of the shoe- 

 maker's measure. Neither the plan, adopted by Forbes and Wright, 

 of using fractions of the English inch, nor that of A. d'Orbigny, who 

 estimates the breadth and height as so many hundredths of the length, 



