2 



BKITISH FOSSILS. 



The body varies in outline from broadly sub-cordate to sub- 

 orbicular, and from depressed to spheroidal. The ambulacra are 

 regularly radiate and linear lanceolate, the apices of the three 

 anterior ones converging at some small distance from those of the 

 two posterior ones, which meet at their tips very closely. The 

 breadth between the avenues of pores is nearly equal in all, but 

 the breadth of the double series of ambulacral plates is greater in 

 the lateral ones than in the odd or anterior ambulacrum. Of the 

 interambulacra, the hinder or odd one and the antero-laterals are 

 narrower than the postero-laterals. In the uppermost portion of 

 each ambulacrum the plates are broad and very sliort, and five of 

 them go to the breadth of an interambulacral plate ; but about 

 half way down the sides they widen out, and near the turn of 

 the base not more than two equal an interambulacral plate. In 

 the dorsal portion of each lateral avenue the pores of each pair" 

 are set well apart and connected by a furrow, but below the middle 

 and in the wider plates they become smaller and closer. There 

 are about 30 or so of the wider pairs. In the odd ambulacrum 

 they are nearly similar throughout. The total number of pairs 

 of pores is between 85 and 50, the number in the odd ambulacrum 

 being less than in the others. The surfaces of the dorsal and 

 lateral plates of all the segments bear very minute equal primary 

 tubercles scattered and intermingled with minute miliary granules 

 on the under surface especially ; on the ambulacral segments in 

 the neighbourhood of the mouth the tubercles become larger. On 

 the anterior portion of the broadly lanceolate ventral posterior 

 interambulacral segment they are closely set and somewhat regu- 

 larly arranged, but become more scattered on its hinder part. 

 On the ventral ambulacral segments there are very few tubercles 

 but closely set granules. All the tubercles are perforated and 

 set upon bosses with crenulated summits. The sides are tumid 

 and rounded in all the varieties ; the cheeks rather sv/ollen. There 

 is a shallow depression in front of the mouth terminating the more 

 or less marked anteal sulcus. The mouth itself is transversely 

 oblong ; its upper lip is higher than the lower, and thickened. 

 The buccal extremities of the avenues form an irregular and obscure 

 star around it. The anal extremity of the test is obtusely truncate, 

 and in the more cordate examples slightly hollowed out vertically 

 beneath the vent. That orifice is placed at about or a little lower 

 than half the height of the test. It is vertically elliptical. The 

 apical disk is much elongated ; it is composed of four perforated 



