Yol. 55.] DR. A. W. EOWE ON THE GENUS MICRASTER. 537 



Micraster prcecursor, according to the zone in which it occurs, has 

 already been made clear. The writer has not seen the type of 

 31. intermedius,'B\iG8ii[le,^'bui the description and figure of this urchin 

 make it appear that it is merely a broad form, with a curved slope 

 from apex to anus, derived from the lower half of the zone of 

 M. cor-anguinum. Its mouth placed far from the margin, and its 

 rudimentary labrum, give colour to this view. Ic is not a true 

 low-zonal form, but, as its name implies, an intermediate stage 

 between the broad low-zonal and the broad high-zonal series. Its 

 well-marked fasciole, and its superior surface with a curved fall 

 from apex to anus, take it out of the Epiasier-senes. This same 

 curved fall from apex to anus is not infrequently seen in the high- 

 zonal Graveseud urchins, both narrow and broad, but in them the 

 mouth is generally nearer the border, and the labrum more developed. 

 The relationship to M. decijoiens, Eayle, M. turonensis, Bayle, and 

 3£. Gosseleti, Cayeux, has already been noted. 



Passage -forms. — The passage-form which is of chief interest 

 is that which occurs in the middle of the 31. cor-anguinum-zone, 

 where, like 3f. prcecursor, it begins to take on high-zonal attri- 

 butes, foreshadowing the full high-zonal development. At this 

 point the horizoual influences become so strong that it ceases to be 

 31. cor-testudinarium, and becomes merely the var. latior of the 

 true 31. cor-anguinum auctorum. 



So far as breadth is concerned, there are passage-forms from 

 31. prcecursor to its broad variety, and this occurs in every zone in 

 which 3[. prcecursor is found, and in every profile-form in which it 

 exists. In fact, there are many forms of Micraster where the eye 

 is unable to decide whether they must be placed in the broad or 

 the narrow series, and the millimetric gauge has to settle the point. 

 It is splitting hairs indeed when a measurement of 1 or 2 milli- 

 metres has to decide the question of a 3Iicraster having one or other 

 specific name. This alone, without taking into consideration the 

 fact that the special features of the test are the same in broad 

 and narrow forms, is sufficient to make one very shy of adopting 

 M. cor-testudinarium as a well-defined species. 



It seems to be beyond doubt that in each zone, and in each species 

 and variety, there are broad and narrow forms ; and it appears to 

 be equally clear that the percentage of broad forms increases as we 

 ascend the zones. Below the Holaster planus-zone there are no forms 

 where breadth and length are exactly equal, but many examples 

 run it very close. It is merely because length is the preponderating 

 factor that we do not have the so-called M. cor-testudinarium in the 

 Terehratulina gracilis-zone. 



Young state. — There is nothing special about this stage, save 

 that the ambulacra are generally shallower and the test more 

 tumid. 



It has been stated that examples of the sub-group 3f. cor-testudi- 

 narium in no way differ from those of the group of 31. prcecursoVf 



1 Bucaille, Bull. Soo. g6ol. Norm. vol. viii (1883) p. 31. 



