^^^' 55*] ^^" ^' ^- ^OWE OlSr THE GENUS MICRASTER. 539 



(7) Sub-group of Micr aster cor-anguinum auctorum, 

 var. latior. 



This, the broad series of Leske's tj^pe, has been included in the 

 present paper, because the zoological scheme would be incomplete 

 without it, and it has just as much title to notice as the broad series 

 of the low zones. Indeed, if numerical proportion have any value, 

 it has a stronger claim, for out of 500 well-preserved and well-grown 

 examples from the upper part of the l\I. cor-angicinum-/.one, no less 

 than 50 per cent, were as broad as long, or broader than long, and 

 this in spite of the increased rostration and carination due to their 

 horizon. 



The broad forms of the high-zonal series have quite as many 

 profile-variations as the narrow series, as will be seen on glancing 

 at PI. XXXV, line vii, where we find shapes recalling those in the 

 line below — the very depressed forma Noinnannice, the flat-arched 

 forma planidorsaia, the round-arched forma beonensis, the forma 

 carinata, and the forma glbhosa. 



The general characters of the test are the same as those given 

 for Leske's type on p. 538 ; and the only way in which they differ from 

 it, apart from breadth, is that they, being broad forms, tend to become 

 large, and that a certain number of them exhibit a tendency to be 

 flat, to have a shallow notch, deeper ambulacra, and a mouth more 

 distant from the border. It is nothing more than a tendency, and 

 these features are absent in not a few broad examples of very large 

 size. 



This tendency is merely the outcome of the rule that most broad 

 forms assume these features up to the top of the 31. cor-anguinum- 

 zone, but above that level they conform more rigidly to the high- 

 zonal type. It has been mentioned before that, in spite of this 

 tendency, so powerful was the influence of life-conditions at this 

 horizon that the broad forms at this level are often elevated, strongly 

 keeled, have shallow ambulacra, a deep notch, and a mouth close 

 to the border. It would seem as if the broad forms tried to pre- 

 serve their low-zonal Goldfussian features, but that the conditions 

 of life swamped them. In a word, while we have an echo of 

 M. cor-testudinarium^ Goldfuss, we have at the same time a fore- 

 shadowing of the well-known broad form of the Belemnitella-Gh2l\. 

 Allusion is made, of course, to M. gluphus, Schltiter, which is a 

 very large, broad, flat form, with angular ambitus, very deep notch, 

 very anterior mouth, and projecting labrum — the culmination of all 

 high-zonal attributes. 



In the M. cor-migiiinum-zone we find examples which almost tally 

 with this last description, but not entirely ; just as we find specimens 

 which resemble M. cor-testudinarium in some features, but which 

 fail to coincide with it in others. The angulation of the ambitus is 

 decidedly a high-zonal feature, but it is largely a condition of full 

 size. 



This sub-group has been styled the var. latior, because that name 

 exactly illustrates the point which it is desired to bring out — 



