336 MICRASTER 



formation, and good examples of the common ty[)e-forms of the Urchin having been sent 

 from France and Belgium for comparison with English specimens, it soon became evident 

 that we had an abundance of the form in our Cretaceous strata, and that with us, as on 

 the Continent, the species characterised a well-marked zone in the Cretaceous formation. 

 I am indebted to Caleb Evans, Esq., F.G.S., for a scries of good specimens from different 

 Sussex and Surrey railway-cuttings, to illustrate the stratigraphical distribution of this 

 species ; and in order that I might figure the ordinary forms of the true German types 

 of Mic. cor-testudmarium, I applied to my friend, Herr C. Struckmann, of Hanover, a 

 distinguished palaeontologist, to send me good types of this Echinide to enalile me to 

 give figures of it for reference ; and I beg to thank my friend for four beautiful examples, 

 one of which I have figured in PI. LXXVI, fig. 1 a—f, for comparison with the British 

 forms obtained from sections on the Surrey and Sussex Railway, near Purley and Kenley, 

 villages between Croydon and Caterham, and likewise from Dover. 



It is a very difficult matter to describe the points in which Micraster cor-angmnum. 

 differs from Micraster cor-festudinarium, Micraster breviporus, and Micraster cor-bovis, 

 inasmuch as these four forms blend into each other by intermediate shapes, which may 

 be obtained from a number of specimens of each group ; and yet typical examples of the 

 four forms when placed alongside each other, seem sufficiently distinct to the eye of the 

 observer to justify the separation of the species. 



In Micraster cor-testudinarium, the test, in general, is flattened on the upper surface, 

 and the ambitus from the anterior third backwards tapers much towards the posterior 

 border, so that the shell has an elongated pyriform shape not much inflated at the sides. 

 In some specimens with an elevated upper surface the test assumes a conoidal outline, as 

 seen in PI. LXXVI, fig. 1 b. In some a prominent ridge extends from the apical disc to 

 the periprocte (fig. 1 a and fig. 2 a), which is absent in others ; in all, the anteal sulcus is 

 shallow and only slightly grooves the anterior border (fig. 1 a and fig. 2 a) ; the apical 

 disc is likewise nearly always central and lies in a depression at the summit (fig. 1 a, 

 fig. 2 a). 



The pairs of petaloid ambulacra are of unequal length, the antero-lateral pair is one 

 third longer than the postero-lateral ; they are not flexed, and extend straight and obliquely 

 outwards at an angle of 40° (fig. 1 a and fig. 2 a) ; the poriferous zones have very 

 small holes arranged in pairs, with a double row of granules extending vertically between 

 the two rows, and forming, in the petaloid portion of the area, a well-marked boundary to 

 the central suture, which is very much depressed. The non-petaloid portion of the area is 

 formed of rhomboidal plates which are articulated with the large interambulacral series 

 (fig. 1 d) ; the single ambulacrum filling the anteal sulcus is narrow, the pores are placed 

 closer together, and the plates are flatter, and have an imbricated appearance in many 

 specimens (fig. 1 «, fig. 2 a). The non-petaloid portion consists of longer rhomboidal 

 plates with the pores situated widely apart and visible down to the central sulcus 

 (fig. 1 d). 



