338 MICRASTER. 



mouth-opening and the width of base ; fig. 1 d shows three large interarabulacral plates, 

 with numerous irregular rows of tubercles on each set in an abundant microscopic 

 granulation, with parts of the petaloid and non-petaloid portions of the arabulacral area 

 magnified to show the wide-set disposition of the pores in the poriferous zone in the 

 petaloidal, and their very different condition in the nonpetaloidal parts ; fig. 1 e shows the 

 size and distribution of the smaller tubercles, and fig. 1 / the size and structure of the 

 larger tubercles, where each tubercle is seen encircled by a smooth areola, with a circle 

 of granules disposed around the margin thereof; fig. 2 ^^ exhibits the structure of the 

 apical disc, and shows the four ovarial plates with very large holes, having the five 

 ocular plates arranged alternately with the ovarials, and the large madreporiform body 

 covering the central portion of the disc. A third specimen, which resembles very much 

 the one I have figured, was collected from Othfresau near Liebenburg. The fourth 

 specimen resembles the breviporus variety, and was found at Langelsheim, near Gosler, 

 Hanover. 



When we compare Herr Struckmann's specimens with the figures in Goldfuss's 

 ' Petref. Germanise,' pi. xlviii, fig. 5 a, b, we see that the type specimen was shorter, 

 broader, and more globular than the Hanoverian Urchins, and that the specimen, fig. 5 d, 

 of smaller size, very much resembles the English Urchins from Purley figured in 

 PI. LXXVI, fig. 2 a, of this work. The types figured by Prof. Goldfuss were collected 

 from the Chalk of Westphalia, and other examples are recorded from the White Chalk of 

 Maestricht and Quedlinburg, and from the Hard or Lower Chalk of Coesfield. 



In Belgium, France, and Germany two zones of Chalk have been recognised by 

 Continental geologists, characterised respectively by Micraster cor-ant/uinum and M. cor- 

 testudinarium, the former characteristic of our Charlton Chalk, and the latter of the 

 Dover Chalk ; and these, it has been shown by Mr. Meyer, F.G.S., and Mr. Caleb Evan.s, 

 F.G.S.,^ who have paid special attention to this subject, exist in England. 



In 1876 Dr. Charles Barrois published a very valuable memoir on " le Terrain 

 Cretace Superieur de V Ancjleterre et de VIrlande" in which he described the Chalk of 

 the South Downs and the Hampshire Basin, and most other parts of England and 

 Ireland, and has clearly shown that it is possible to correlate the zones of life in the 

 English Chalk with corresponding zones which he has already established in the Chalk of 

 France. Mr. Caleb Evans contributed to the Geologists' Association in 1877, a valuable 

 paper,^ with figures of species, " On the Forms of the Genus Micraster common in the 

 Chalk of West Kent and East Surrey," in which he pointed out the distribution of the 

 species in these zones. 



The classification which Dr. Barrois adopts is shown in the following Table, to which 

 I have added the English equivalents of the same as identified by Mr. Caleb Evans. 



1 In the lettering of this plate the numerals 1 d have been twice repeated ; that of the apical disc 

 should be 2 d. 



- 'Proceedings of the Geologists' Association,' vol. v, No. 4, p. 149, 1877. 



