I'KXTACEROS lirLBTI-'KRUS. 70 



iufero- and supero-margiiial plates appear in dorsal view. In fact, the infero- 

 marginal plates do not take any part in the formation of the actinal surface. This 

 is parallelled in niodoru species of Pentacerot^, e.g. P. clarattis. In marginal view 

 the supero- and infero-marginals at the extremity of the ray very distinctly 

 alternate. This alternation persists at the base of the arms, hut here it is not 

 always so obvious. 



The supero-marginals are much higher than the infero-marginals, and also more 

 oblong in shape. Both supero- and infero-marginal plates are regularly but 

 coarsely pitted for granules. 



In spechnen figured on PI. XX, fig. 1, we obtain the following measurements: 

 Breadth of fifth infero-marginal from the extremity of ray . -j'S mm. 

 Lenj^th „ „ „ „ „ .4 



Breadth ,, supero-marginal „ ,, „ . 9"2 ,, 



Length „ „ „ „ „ . 3-9 „ 



Breadth „ radialia „ ,, ,, . 5"8 „ 



Length „ „ „ „ „ . 4-4 „ 



Width of ambulacral groove . . . . . 1-2 „ 



The ocular is visible in this specimen. It is about 1"0 mm. in length and 

 breadth. The extremity is slightly pointed, and its ventral surface is hollowed out. 

 A ventral view is figured on PL XXI, fig. L Ventro-lateral plates extend 

 almost to the extremity of the arms. These are, as usual, rather greater in breadth 

 than in length. The ailambulacral plates appear to be about half the length of 

 the bordering actinal plates. Their armature consists of several rows of spinelets 

 arranged in pairs. 



A few intermarginalia are present in the interradii. They, as usual, press the 

 supero- and infero-marginalia on to the aljactinal and actinal surfaces of the disc 

 respectively. 



LocalUy and Straiigyaphical Position. — Upper Chalk, Bromley, Kent; according 

 to Dr. Rowe, probably from the Chislehurst caves near that locality. 



Variations. — Variations occur amongst all the si)ecimens, especially with 

 regard to the ornamentation of the plates and the madreporite. The British 

 Museum specimens, 48748, which occur together in a slab, are especially note- 

 worthy, inasmuch as the lowest situated individual possesses on the disc no plate, 

 which is bnlbiforni or raised con.spicuously above the remainder. 



