Vol. 55.] DE. A. W, EOWE ON THE GENUS MICKASTER. 517 



It must be remembered that in the lower 80 feet of the zone of 

 Micraster cor-anguinum we reach the point where the high-zonal 

 series begins to divide from the low-zonal series. Without the aid 

 of the special features of the test it would be impossible to dis- 

 tinguish M. jproecursor and M. cor-testudinarium, as found in the 

 base of this zone, from those found in the zone below. 



At this point it will be profitable to contrast the features of the 

 test, as shown in the low-zonal and high-zonal series (see Table on 

 the opposite page). Only the broad features are set forth, and the 

 reader is referred to the Tabular Summary (pp. 512-513) and to § II. 

 for a more minute description of the essential zonal details. 



It will, therefore, be noted that the low-zonal forms are the very 

 -antithesis of the high-zonal, of which M. cor-anguinum auctorum 

 is the type ; and it is abundantly clear that the specific types, how- 

 ever marked and prominent they may be, are merely accentuated 

 examples of certain characters seen in forms from definite horizons. 

 This is especially noteworthy in the case of M. cor-testudinarium, 

 for the very features which Goldfuss, in his ' Petrefacta/ sets forth 

 as diagnostic of the species — shallow notch, deep ambulacra, and 

 mouth distant from the border — are common to all varieties and 

 species as far up as the top of the lower third of the 31. cor-anguinum- 

 zone, breadth alone excepted. 



No attempt will be made in the present paper to deal with all the 

 Continental forms described as occurring in these zones, as this 

 description is essentially one of a typical English series. More- 

 over, certain Continental forms are absent in England, and others 

 are inaccessible. A few forms described from Continental deposits, 

 which are also found in England, are essential to the illustration of 

 the text, and will be noticed in the description of the Glroups under 

 which they fall. 



lY. Division oe Mich aster into Geotjps. 



For the sake of zoological and zonal interest I propose to divide 

 Micraster into Groups. Bearing in mind the plasticity and 

 abundance of the genus, it would seem that this is the only clear 

 course to adopt, as by this means alone can the zoological sequence 

 und the trend of evolution be traced. True species, and even 

 prominent varietal types, are rare, and passage-forms and trivial 

 variants are the rule. Nothing but a Group will embrace them 

 all, and give to each series its correlative value. As this paper is 

 not a monograph on the genus, a synonymy will be dispensed 

 with. 



The Groups employed will be as follows : — 



1. Qromp of Micraster cor-bovis. 



2. Group of M. LesJcei. 



3. Group of M. prcecursor (the low-zonal type). 



4. Group of M. cor-anguinum auctorum (the high-zonal type). 



It wiU be noticed that it has not been thought necessary to 

 constitute a Group of M, cor-testudinarium, any more than one of 



