318 Dr. J. W. Gregory — A new Liassic Eehinoid, 



Test small, turban-shaped. 



Apical system large, area equal in width to half the diameter of 

 the test ; arrangement of plates unknown. 



Ambulacra of simple, primary plates near apical area; of com- 

 pound plates formed by union of three primary plates at 

 ambitus. The poi - e-pairs are in a straight, single series. 

 Ornamented by granules and a small primary tubercle on 

 some of the ambital plates ; the primary tubercles ore 

 perforate and non-crenulate. 



Iuterambulacra of plates which, in comparison with the size of 

 the test, are high and large; the plates are unituberoulate, 

 each bearing a single, primary tubercle, which is perforate 

 and non-crenulate. 



Distribution. — Lias : England. 



Type Species. — A. Thompsoni, n.sp. 



Affinities. — This genus is based on some small but admirably 

 preserved specimens from the Upper Lias of Higham Ferrers, in 

 Northamptonshire. They were collected by that indefatigable worker 

 on the Midland Lias, Mr. Beeby Thompson, F.G.S., by whom they 

 have been presented to the British Museum. 



It is clearly a member of the family Diadematidaa as defined by 

 Duncan. Of the four subfamilies of that great group, it differs from 

 the Orthopsinse by having compound plates in the ambulacra, and 

 from the Diplopodinee and Pedininaa by having the pore-pairs in 

 simple vertical series, instead of their being biserial or triserial. 

 The specimens agree, on the other hand, in all points with the 

 Diadematinae. Among this subfamily the new genus is most nearly 

 allied to the genus Diademopsis, from which it differs by two 

 characters : 1st, the pores in Diademopsis are either crowded or occur 

 in arcs near the peristome ; 2nd, the interambulacral plates in the 

 same genus are lower and broader, and each bears two primary 

 tubercles, which are lower and smaller than those of this new form. 

 It differs from Semipedina by the same characters, for the intimate 

 affinity between this genus and Diademopsis'\s now generally admitted. 

 In some respects Archceodiadema resembles Microdiadema, Cotteau, 

 from which, however, it differs by three very definite characters : 

 1st, the apical area is equal to half the diameter of the test, instead 

 of about one-fifth of the diameter ; 2nd, the tubercles are perforate 

 and non-crenulate, instead of perforate and crenulate ; 3rd, there is 



