Yol. 55.] DR. A. W. JROWE ON THE GENITS MICEASTER. 52^ 



smooth interporiferous area, with inner ambulacral rows frequently 

 elongated, rudimentary interporons ridge, marked anterior excen- 

 tricity of disc, very shallow sulcus and notch, mouth distant from 

 the border, similar labrum, undivided peristome, rudimentary tuher- 

 culation of lahral plate, and feeble fasciole. The points of distinction 

 are that in Micraster Leslcei the size is small, the test thick, the 

 areolar circle and inter-areolar granulation round the primary 

 tubercles of the base are strong, the periplastronal area granular, and 

 the carina frequently moderately developed — all of which features 

 are absent in M. cor-bovis. In addition, the anterior segment of 

 the peristome is an undivided raised ring in 31. Leskei^ whereas it is 

 flat in M. cor-hovis. With these points of distinction to guide one, 

 it will be impossible to confound the two species. 



Variations. — As in every other Micraster, these are con- 

 spicuous, and while the essential features of the test are unalterable, 

 there is considerable difference in the less important, and more 

 plastic, characters of mere size and shape. The variations in size 

 and shape are depicted on PI, XXXV, line i, Nos. 1 to 4, and the 

 forms shown are the depressed forma Normannice, the flat-arched 

 iorvaa planidorsaia, the forma carinata, and the forma gihhosa. On 

 the extreme right of line ii is included one of the large forms, of 

 characteristic shape and dimensions. 



Young state. — The young forms show no deviation in the fixed 

 characters of the test, but they are generally more inflated, with 

 shallower sulcus and notch, and with shallower ambulacra. The- 

 relatively small size of the mouth and of the primary tubercles is 

 not so conspicuous as it becomes in the mature state. 



Pass age -forms. — This is a point of extreme interest, as it 

 marks the continuity of the evolution of the genus in its more 

 primitive forms. From the Holaster 2^lanvs-zone both Gen. Gockburn 

 and the writer have obtained examples of medium size, where the 

 test is thin, and the interporiferous area is either ' smooth ' or faintly 

 * sutured,' and yet where the other fixed features of the test lean 

 more towards M. LesJcei. Every possible variation between the two 

 species may be traced in their passage-forms. Por instance, in 

 PI. XXXVIII, fig. 5, is shown an example, in beautiful preservation, 

 where the ambulacra are typical of M. cor-hovis, and the peristome 

 is also of that type, and yet the granulation of the periplastronal area 

 and around the basal tubercles essentially belong to ili. LesJcei, 

 though the test is as thin as in a specimen of H. planus. Another 

 example has a typical M. Zfs7l:e^- peristome, and yet the other 

 features of the base are characteristic of M. cor-hovis. Here again 

 the test is quite thin. These variations could be multiplied, and 

 nothing but the exigencies of space prevents me from giving 

 photographs of these instructive transitions. 



It should be noted that no other thin-tested Micrasters have 

 been found, either in the H. planus or M. cor-testuclinarium-zone^, 

 which have the ' inflated ' interporiferous area, and the other 

 characters associated with M. prcecursor in these zones. This is of 

 manifest importance, as it goes far to prove that M. cor-hovis passes 



