510 DR. A. W. ROWE 01^ THE GENUS MICRASTEK. [Aug. 1 899, 



line i. M. cor-bovis lto4;M. Leskei 1' to 5'. ] 



line ii. M. pnecursor (see p. 531). I Zone of Holaster planus. 



line iii. M. cor-testudinarium. J 



line iv. M. prcscursor. ") _ „ ^^ ^ _ 



line V. M. cor-testudinarium. j ^°^^^ <^^ ^' cor-testudmanum. 



line vi. M. cor-anquinum. ") „ „ ,^ 



line vii. M. cor-anguinum var. latior. ] ^^^^ ^^ ^- cor-angmnum. 



This plate is intended to show the contrast, and, at the same 

 time, the continuity and affinity, which exist between the low-zonal 

 and high-zonal series ; it is also intended to .indicate the similarity 

 in profile afforded by the broad and narrow forms in each zone. 

 The dominant profile-shapes may be enumerated as follows : — 



(1) The very depressed form, with either straight or sloping posterior 



truncation — No. 1 in each line. This is the forma Normannia. 



(2) The less depressed form, -with flat-arched upper surface — No. 2 in each 



line. This is the dominant po'CBCursor-iorra in the low-zonal series — 

 the forma planidorsata. 



(3) The round- arched form, always a tall example, with carina forming a 



curved posterior fall — No. 3 of each line. This is the forma heonensis. 



(4) The form with a definite posterior rise from the apex to the posterior 



truncation, owing to the development of the carina — No, 4 of each 

 line. This is the forma carinata. 



(5) The gibbous form, with a straight fall from the apex to the posterior 



truncation. This is a very interesting form, and it is worthy of note 

 that every example here figured has a strong sub-anal fasciole. This 

 is shown in No. 4 of the M. cor-bovis series, and in No. 5 of all the 

 other lines, including No. 5' of the M. Leskei-gvouip. Nos. 6 & 7 also 

 belong to this class, and are added to show the varying degi-ee of the 

 gibbous slope. It clearly indicates that there is no need for a gibbous 

 Micraster to be a tall one. This is the forma gibbosa. 



It will thus be seen that every zone, and every species and 

 variety in that zone, shows a definite scheme of profile-shape in its 

 micrasters, and that this scheme of shape may be faithfully repro- 

 duced in neighbouring and allied zones, and be strongly reproduced 

 in the more distant and higher zones. 



Though the general scheme for the high zones is seen to be the 

 same as that in the low zones, there are certain added features to 

 be noticed, such as the sharp anterior slope, the strong rise of the 

 carina, and the development of a rostrum. These features aife 

 merely the outcome of a persistent evolution, and they in no way 

 detract from the complete parallel afforded by the two series. 



It would have been easy to add examples of other profiles, 

 such as those shown in M. Bucailli, Parent, and M. turone^isis, 

 Bayle, and even of other prominent shapes noted by the writer; 

 but this would have resulted in a considerable enlargement of the 

 staging, and a consequent reduction in the size of the specimens 

 figured. In any case, the five forms figured are incomparably the 

 commonest, and they amply suffice to give point to the contention 

 herein advanced. 



It is, then, abundantly clear that mere shape is the feeblest reed 

 to lean upon in constructing a specific type, and that unless an 

 urchin has other special features, which single it out from its fellows 

 in any given zone, there is no ground for founding a new species. 



