Yol. 55.] DE. A. W. EOWE ON THE GENUS MICRASTER. 511 



It is only by grouping the special features of the test that we can. 

 obtain any idea of the zone from which an example is derived, and 

 it is only by the aid of these same special features that we can 

 hope to arrive at the definition of a group, or what is infinitely 

 more rare, a true and constant species. Thus, if in the zones of 

 Holaster planus and Micraster cor-testudincn'-ium, for instance, where 

 deep ambulacra, an anterior disc, a posterior mouth, a feeble labrum, 

 a thin sub-anal fasciole, and a shallow ambital notch are the in- 

 variable rule, we were to find a depressed form which had shallow 

 ambulacra, a central disc, an anterior mouth, a prominent labrum, 

 a strong fasciole, and a deep ambital notch, we should be justified 

 in giving it a specific title, in order that we might note the 

 divergence from the dominant horizonal type. Anything short of 

 this is a mere variety, unless, as in the case of M. cor-bovis, in 

 addition to the zonal features we find other superadded features 

 which are peculiar to that type, and to that type alone. 



Angulation of ambitus. — This is an uncommon feature m 

 low-zonal forms, and is only present, as a rule, in large, broad, and 

 fiat examples. It is more common in the high -zonal series, but is 

 still confined to the same advanced and mature types, and this 

 accounts for its presence in 31. gh/phus, Schliiter. 



Now, granting that this detailed analysis of the separate features 

 of the test is accurate, it may reasonably be asked whether these 

 features afford sure guidance in determining the zone from which 

 any given Micraster is derived. Purnished with this knowledge of 

 detail, can we go into a quarry, or attack a cliff-section, pick 

 out a few specimens of Micraster, and from this evidence alone 

 decide the zone in which we are working? This is a severe 

 test, and one would not employ it in the field to the exclusion of 

 other collateral evidence of associated fossils ; but severe as the 

 test may be, it is both practicable and effective. Por instance, it 

 is always possible to fix the junction-line of the zones of H. planus 

 and M. cor-testudinarium by the form of the ambulacra alone, and 

 it will be found that the level at which the 'sutured' and gently 

 'inflated' interporiferous area ceases is coincident with the disap- 

 pearance of the guide-fossils If. planus and M. cor-bovis, and there- 

 fore with the incoming of the M. cor-testudinarium fauna. Both 

 H. planus and M. cor-bovis may be absent or rare, and then we 

 should have to fall back on the gasteropods, which may also be 

 absent ; so that the Micraster is left to us as the only fixed quantity 

 upon which we can rely. 



Though the ambulacra are generally sufficient in themselves as a 

 guide, an inspection of the detailed analysis and of the Tabular 

 Summary (pp. 512-513) will show that there are other features of the 

 test which one can join to the interporiferous area, and so make the 

 determination more complete. Attention is called more particularly 

 to the arrangement of tubercles on the labral plate, the length of the 

 labrum and the nature of its tip, the conformation of the peristome 

 and the degree of granulation of the periplastronal area. 



