^^^' 55'] ^^* ^' ^' T^OVfE ox THE GENUS MICRASTER, 503 



ambulacra, for the reason that the features are not, as a rule, suffi- 

 ciently prominent or well preserved to warrant one in burdening the 

 mind with unprofitable detail, which cannot be used in the field. 



The conclusions which one may draw from these observations 

 on the interporiferous area are clearly brought out in. the per 

 centages quoted. It will be seen that Micraster cor-bovis and 

 M. Leskei alone have the smooth central area, aad that this, together 

 with the sutured and gently inflated type, is distinctive of the 

 Eolaster planus-zone ; that the strongly inflated and the subdivided 

 areas are characteristic of the zone of M. cor-testudinarium and of 

 the base of M. cor-angidnum ; while the divided area is typical 

 of the upper part of the M. cor-anguinum-zonQ. 



When we come to deal with the special features of the various 

 ' species,' it will be evident that in the interporiferous area we have 

 a reliable guide to horizon. It will be shown that there is no 

 difference in the general facies of M. jprcEcursor and M. cor-testudi- 

 narium in the zone of H. planus, or in the same forms when they 

 occur in the zone of M. cor-testudinarium, nor do these forms occur 

 more frequently in one zone than the other ; and yet a glance at the 

 interporiferous area will reveal with practical certainty from which 

 bed each form is derived. Further, this area is the chief means of 

 marking the passage-forms from one species to another, and therefore 

 of showing the unbroken continuity in the evolution of the genus. 



Length and breadth. — In the Terebratulina c/racilis-zone we 

 find chiefly long forms, and these are notably longer than in any 

 other zonal series oi Micraster, on account of the preponderance of 

 M. cor-hovis ; and while, even in the case of M. cor-bovis, there is a 

 tendency to establish a broad and a narrow form, the breadth never 

 actually equals the length, as it not uncommonly does in other 

 species in the zones above. 



In the other low-zonal beds the narrow forms strongly pre- 

 dominate, and it is only when we reach the high-zonal series, in the 

 upper part of the M. cor-anguinum-zone, that we find an equal 

 number of broad and narrow forms. It appears to be beyond doubt 

 that in each zone there are broad varieties of the narrow forms, 

 and that the proportion of broad forms increases as we ascend in 

 the Chalk-zones. 



Height. — All low-zonal forms, with rare exceptions, are de- 

 pressed, and it is not until we reach the base of the M. cor-anguinum- 

 zone that elevated forms begin to be strongly represented. Above 

 that limit tall forms are the rule, but very broad and large examples 

 are not infrequently depressed. There seems to be a marked ten- 

 dency, even so high up as the top of the M. cor-anguinum-zone, for 

 the broad forms to be depressed, and thus suggest features which 

 have always been considered characteristic of M. cor-testudinarium ^ 

 Goldfuss. 



