FROM THE UPPER CHALK. 327 



cheeks. In other respects its form and structure has many affinities with Ecldnocorys on 

 the one side and Holaster on the other, between which it forms a connecting link. 



Locality and Stratiyraphical Position. — It is found in the upper chalk of Kent, 

 Sussex, and other Enghsh counties. 



In France it is collected from the upper chalk at Meudon, near Paris ; Dens, Poigny, 

 Yonne ; Beauvais, Oise ; Saintes, Charente-Inf^rieure. 



In North Germany it has been found at Peine, at Yseburg, near Hanover. 



History. — This Urchin was first described by Lamarck as Ananchytes 2nllula, and after- 

 wards by Deslongchamps under the same name. Desmoulins removed it into the genus 

 Spatanyus,'RoQmeT described and figured it ?iS Ananchytes analis;Agassiz andDesor removed 

 it into the genus Holaster. Forbes called it Ananchytes {Holaster) jnlhila, and described 

 and figured it in his Memoirs of the Geological Survey in much detail and with great 

 accuracy ; and finally D'Orbigny, having ascertained the presence of the fasciole which 

 extends along the margin and under the posterior border, removed it into the genus 

 Cardiaster, among its marginal fasciolated congeners. 



Ge7ius — EcHiNOCORTS, Breyiiius, 1732. 

 Ananchytes, XamarcA, 1801. 



Form of the test more or less oval ; upper surface much elevated, helmet-shaped, 

 convex, conoidal, or rounded ; under surface flat. 



Ambulacral summit central, the poriferous zones occupy the middle of the ambulacral 

 plates ; the single ambulacrum similar in structure to the antero- and postero-lateral 

 pairs. The pores round or elongated, alike identical in structure, disposed in single 

 pairs, set obliquely widely apart in the middle of the ambulacral plates, and forming two 

 rows in each area, which radiate from the summit and diverge towards the circumference. 



Apical disc elongated, formed of four ovarial and five ocular plates. The two pairs 

 of ovarial plates are separated by a pair of oculars. 



Under surface very flat. Mouth-opening near the anterior border transversely oval ; 

 peristome bilabiate, situated in a depression, and surrounded by a stellate arrangement of 

 primary tubercles, which diverge from the circumference of the peristome. 



Vent marginal or infra-marginal ; periprocte small and oval in a vertical direction. 



Surface of the plates covered with small primary tubercles ; those at the base are 

 larger and more developed, some are raised upon bosses with crenulations, and all are 

 perforated at the summit. 



The Echinocorys resemble the Ilolasters, but are distinguished from them in having 

 the single ambulacrum precisely similar to the others, and by the absence of the anteal 

 sulcus. The position of the periprocte is likewise entirely marginal or infra-marginal. 



