PROM THE WHITE CHALK. 337 



The interambiilacral areas are wide, and formed of large plates, eight, ten, or twelve in 

 each cohimii, according to the age of the Urchin. They are largest at the base and diminish 

 in size on the sides and upper surface. The plates are covered with several (four to six) 

 irregular rows of tubercles, nearly uniform in size, and from 20 to 30 on each (fig. 1 d); the 

 surface of the plates is covered with a microscopic granulation, which forms circles around 

 the tubercles. At the base, the tubercles are much larger, especially about the cheeks 

 and anteal sulcus, and on the plastron they are closely clustered together ; they are 

 encircled by smooth areolae, each surrounded by a ring of granules (fig. 1/). 



The subanal fascicle is very distinctly visible in all the specimens that have passed 

 through my hands. 



The apical disc is a small body set in a central depression, and composed of four 

 perforated ovarial, and five perforated ocular plates, all closely welded together, the 

 right antero-lateral ovarial having a small madreporiform body on its surface (fig. 2 d). 



The posterior border is narrow and truncated obliquely downwards and inwards. 

 The vent occupies the upper part of this region, and the oval periprocte opens beneath the 

 termination of the central ridge on the upper surface (fig. c) . 



The base is rounded laterally, and flattened in the longitudinal direction, the mouth- 

 opening is situated at some distance behind the sulcus, the peristome is transversely oblong, 

 and the under lip forms a strong projecting process of the shell (fig. 1 c and fig. 2 b). 



Affinities and Differences. — I have already pointed out (pp. 275, 279) the affinities 

 which this Urchin has with Micraster cor-anguinum on the one side, and with Micraster 

 breviporus and Micraster cor-bovis on the other, and have stated how difficult it is to detect 

 any characters of specific value by which it may be described as distinct from either. I 

 have long considered M. cor-testudinarium as a variety of if. cor-anguinum. The backward 

 position of the mouth-opening, the flatness of the upper surface, and more cordate shape of 

 the test, are not characters of specific value, although they may be useful in establishing 

 the fact that varieties lived under special conditions in the Cretaceous sea, and a separa- 

 tion of the varieties may, therefore, be of value for stratigraphical purposes. 



StratigrapMcal Position. — Mr. Meyer regards M. cor-testudinarium as a characteristic 

 fossil of the Dover chalk, and M. cor-anguinum of the Charlton Chalk which belongs to a 

 much higher bed in the Cretaceous series -^ and this appears to correspond with the 

 horizons in which these forms are found in France and North Germany. 



Herr Struckmann's specimens of Micraster cor-testudinarium, Goldf., were all obtained 

 from the Middle Plsener of Hanover, and very much resemble the English Urchins 

 collected from the cuttings of the Surrey and Sussex Railway, — so much so that, were 

 they not carefully marked, the EngHsh and German forms could not be distinguished from 

 each other. Two of the specimens were collected from the Middle Plsener of Weddingen 

 near Liebenburg, and the largest of these is figured in PI. LXXVI, fig. 1 a, b. The details 

 of the anatomy of the test are given in fig. 1 c, showing the backward position of the 



^ C. Evans " On Forms of the Genus Micraster," ' Proc. Geol. Assoc.,' vol. v, No. 4, p. 3, 1877. 



