ECHINITES. 207 



minute or granular tubercles often run along the line of the suture. 

 The vent is circular, and has a beautiful pentagonal border. The 

 middle of the base is depressed. 



No. 3 is another elegant species of cidaris, which occurs both in the 

 oolite and in the chalk. It appears to be a variety of cidaris niiliaris, 

 or rather cidaris diadema ; coinciding nearly with the specimen given 

 in Plot's Oxfordshire, Tab. V. Fig. 5; but more exactly with that in 

 Parkinson's Organic Remains, III. PI. I. Fig. 4. Its tubercles are of 

 the same form as those of No. 1, and disposed in a similar way; but 

 instead of having five double rows very large and five very small, it 

 presents us with five double rows of moderately large tubercles, and 

 five double rows only a little smaller and closer, alternating with 

 them. Ten double rows of pores pass between these ten areae, as in 

 No. 1 ; but they run nearly in straight lines. We also perceive nu- 

 merous small tubercles, disposed between the larger mammillge, and 

 sutures dividing the shell into compartments; but with less regularity 

 than in No. 1. This fossil too is considerably flatter than the other, 

 and its base is not quite circular, the five smaller arese projecting a 

 little beyond the broader area;. The vent in this specimen is not 

 distinct; but it has no doubt been pentagonal. 



Figure 10 is the cidaris corollaris, corresponding Avith that in 

 Parkinson's Organic Remains, ID. PI. I. Fig. 7; which is considered 

 as a siliceous cast. This specimen like that, has a knob of flint above 

 and another below, concealing both the vent and the mouth; the 

 result, as Mr. Parkinson thinks, of an excess of siliceous matter, 

 beyond what was required to fill the shell. The markings of this 

 fossil, which is from the alluvium, are elegant. Like No. 3, it is 

 divided into five larger and five smaller areae, parted by lines of 

 pores; whicli lines are only double towards the base, but quadruple 

 on the upper surface. Tlie lesser spaces, which bear but a small 

 proportion to the greater, present two promiuent ridges witli a furrow 



