2 



BRITISH FOSSILS. 



The primary tubercles on tlie ambulacra are ai-raiiged in two 

 rows along the margin, and are small and inconspicuous ; there are 

 about thirty-two on the upper face. Between these are two other 

 rows, less regular, but reaching halfway up, or sometimes further, 

 and near the margin one or even two rows are introduced within 

 these. On the under side the tubercles increase much in size, and 

 are in four regular rows. The areolae are very flat and depressed 

 round their margin, which is often faintly radiated ; there is no 

 appearance of crenulation on their upper edge. The boss is small 

 and bead-like, and shows the perforation well. There is a circle of 

 granules round each areola, and scattered equal-sized granules, 

 without any secondary tubercles between, over the surface. 



The avenues are narrow and rather sunk, the pores in very 

 regular single file (not at all undulated as in P. semisulcatus.) 

 Each pair consists of a transverse and an oblique one, not differing 

 much in size, and separated by a small prominent ridge, not a 

 tubercle. There are about three ambulacral plates to each primary 

 tubercle, so that there are nearly ninety plates in a series on the 

 upper face. The pairs of pores are a little more distant below. 



The interambulacral tubercles are like the others in size on the 

 upper and lower faces ; and their arrangement is very similar to 

 that in the last-described species, viz., in transverse rows on the 

 inner half of each ambulacra! plate, and in scattered order on the 

 outer portion, where they are decidedly larger, and, as our fig. 5 

 shows, often irregular ; and sometimes they are absent. On the 

 marginal plates there are often five tubercles in a row on the inner 

 half, and three double rows on the outer, but above they are much 

 fewer and less regular. About seventeen plates occur in an inter- 

 ambulacral series, much bent in outline, as shown in figure 5. The 

 primary row {a) of tubercles occupies the median angle as usual. 



The tubercles on the under surface are too wide apart in our 

 figure. They are in irregular transverse rows, and their smooth 

 depressed areolae in some parts nearly touch, leaving room only for 

 the granules between. "We can see no trace of crenulation on the 

 edges. 



Affinities. — Except with the pyramidal variety of P. semisulcatus, 

 before noticed, there is no species with which this can be compared. 

 The small regular tubercles well distinguish it from that, and, 

 together with the elevated ambulacra, seem to afford a good specific 

 character. They are raised not only in the middle, but from their 

 edges, and the avenues consequently sunk in a shallow groove. 



