W. KEEPING ON THE GENUS PELANECHINTJS. 927 



millim. 



Height of interambulacral plate at equator 4| 



Breadth of ambulacral area at equator 16 



„ ,, „ oral pole 10 



,, large ambulacral plate at equator 4 



„ small ambulacral plate at equator .... 2 



„ poriferous zones 4j 



Length of buccal plate 8 



Breadth of buccal plate 4 



„ exposed part of buccal plate 1 



Length of secondary spines from 3 millim. to 10 



„ primary spines nearly 20 



II. De. Weight's Specimen. 



Like the one just described, this specimen has only the oral aspect 

 exposed, and of this hemisphere about two thirds are well preserved. 

 The jaws are conspicuous, the large and powerful " alveoli " (mea- 

 suring 25 millims. by 10 millims.) being in strong contrast with the 

 delicate nature of the test. 



Only a few of the overlapping plates of the buccal area are scat- 

 tered about, and these, together with the ambulacral and interam- 

 bulacral areas of the test, all agree precisely with the more perfect 

 specimen already described above. 



Some overturned interambulacral plates show that every primary 

 tubercle is represented in considerable detail by a pit on the internal 

 surface of the plate, even the ligament cavity of the tubercle having 

 a corresponding prominence in the internal pit. 



The primary spines (fig. 8) are numerous and larger than those 

 preserved in the Cambridge specimen. They measure 23 millims. 

 The broken ends of these spines show that they were hollow, the 

 cavity being usually filled with a crystalline carbonate of lime, darker 

 in colour than the spine substance. 



But the most remarkable feature of this Urchin is shown on a 

 part of the surface near the mouth where the coronal plates have 

 been removed. Here, instead of the bare oolite rock, we see exposed 

 a series of thin imbricating scale-like plates (fig. 10), which appear 

 to have been internal to the ordinary coronal plates. They are 

 very irregular, both in size and shape, two contiguous ones in one 

 place being equal and similar to each other ; but in another pair one 

 plate is half as large again as the other. Still, in their general 

 size and proportions, they are similar to the overlying interambula- 

 cral plates. Their contours are all rounded or gently undulating, 

 and their surface is simple and marked only by a minute punc- 

 tation. Their thinness is extreme. 



It is not easy to determine what was the natural relation of this 

 set of plates to the rest of the test. They are certainly not internal 

 views of ordinary interambulacral plates, for they are too irregular, 

 they overlap each other, and, moreover, do not exhibit any depression 



