526 DE. A. W. EOWE ON THE GENUS MICRASTEE. [Aug. 1 899, 



Micrnster, and those which have been given to the writer from Con- 

 tinental sources are also rather small. Cotteau, Roemer, de Loriol, 

 Cayeux, and Gauthier, on the other hand, have medium^ or even 

 large, examples in view in their figures and descriptions, and it 

 behoves us to examine their position, and to show where their diffi- 

 culties lie. Cotteau, for instance, distinctly defines it as ' a species of 

 rather large size,' and he even gives measurements of examples (not 

 M. cor-hovis), which are 65 mm. long ; while in addition, he figures 

 in his ' Echinides fossiles de TYonne,' pi. Ixxv, fig. 6, a large tumid 

 example, which has given rise to much discussion by subsequent 

 writers. 



It will be unnecessary to analyse the views of each individual 

 writer, as they all more or less fall in with Cotteau's ideas, in 

 referring all these large forms to M. Leshei, though some of them do 

 so with obvious hesitation. The prevalence of the ' sutured ' inter- 

 poriferous area in the zone of Holaster planus — a purely horizonal 

 feature — and its similarity to the ' smooth ' area in M. Leshei^ have 

 probably induced many writers to refer these large examples to the 

 Micraster in question. 



A solution is possible only in a section like that of Dover, where 

 the beds are very rich in Micrasters, and where the dip is very 

 gentle, thus giving one the chance of working a long clifi'-section 

 yard by yard. In the H. plaims-zone at Dover — and the same occurs 

 at Beachy Head, on the Dorset coast, and at Beer Head — we find a 

 much larger proportion of small forms than in any other zone, which 

 all more or less resemble what one looks upon- as the type of 

 M. LesJfei; and though mere size is, as a rule, the weakest of all 

 evidence to lean upon, we are forced to consider here whether it may 

 not have its meaning. While it is impossible to deny that this zone 

 is peculiarly rich in small forms of Micraster, it is equally certain that 

 we find an abundance of medium-sized forms, as well as some of the 

 largest examples which the English Chalk produces. These last are 

 especially common just above the Chalk-rock at Dover. It is, there- 

 fore, clear that in this zone Micraster had as good a chance of 

 reaching maturity as at any other horizon, and that the small 

 M. Leskei-iorms cannot be considered as mere immature examples. 

 The medium-sized and large forms frequently have an interj)oriferous 

 area, which at the first glance, appears to be smooth, though in 

 reality it is clearly ' sutured ; ' the labral plate, moreover, is freely 

 tuberculated, and the anterior segment of the peristome is plainly 

 jointed. These are the forms, found only in the H. plaaus-zone, 

 which constitute a perfect ti-ansition between M. Leshei and 

 M. prcecursor. 



The only fair way to establish a diff'erence between the small 

 type-form and the larger transition-form is to examine a large 

 series of each by means of photomicrography. This has been done, 

 and in PI. XXXYIII, fig. 1, is shown an average-sized and typical 

 example of what the writer considers to be M. Leslcei ; while in 

 fig. 3 of the same plate is depicted the characteristic peristome of this 

 species, and in PI. XXXVI, fig. 2, is seen one of the anterior paired 

 ambulacra. By way of contrast, the reader is referred to PI. XXXYI, 



