126 PSEUDODIADEMA 



of the inter-ambulacral ; the notches are deep, and their margins are much reflexed 

 (fig. 4 c) ; the base is shghtly concave in many specimens, in others it is almost flat. 



The spines are well preserved in situ in a specimen in the British Museum, from which 

 our figure (PI. XII, fig. 7) is drawn ; the primary spines in that urchin (fig. 7 a, b) are 

 eleven twentieths of an inch in length ; the stem is marked with longitudinal lines, the 

 milled ring is not very prominent, and the spine may be said to be stout in proportion to 

 its length ; the secondary spines are one fifth of an inch in length, and are miniature 

 forms of the primary spines. In some specimens, as one from Steeple-Ashton, kindly 

 presented to me by Mr. Mackneil, of Wootton-under-Edge, the primary spines are swollen 

 out, and slightly bent in the middle of the stem ; the head and acetabulum are large, 

 corresponding with the magnitude of the tubercles. 



Affinities and differences. — This urchin was formerly identified with Cidarites suhangu- 

 laris, Goldfuss, but on comparing the German type of that species with ours, there can be 

 no doubt of their distinctness. Goldfuss describes his species as having six or eight large 

 tubercles in each row, with crenulated bosses in all the areas ; the ambulacra are prominent, 

 and the test is therefore pentagonal ; each tubercle is surrounded with a circle of small 

 graoules. Now Fseudodiadema versipora has twelve primary tubercles in each row in the 

 inter-ambulacral areas : the tubercles are consequently set close together, and all the areolas 

 are confluent, so that each tubercle is not surrounded by a circle of small granules. It 

 resembles Cidarites snhangularis in possessing a double row of pores in the upper part of 

 the poriferous zones, and in having a row of secondary tubercles between the primary 

 tubercles and the pores. Pseudodiadema versipora very much resembles Pseudodiadema 

 suhangulare, Agassiz, which appears to be quite distinct from the Cidarites snhangularis, 

 Goldf. ; in fact, the beautiful figures which Agassiz has given of the Swiss urchin leads 

 me to think that it may be identical with our Pseudodiadema versipora. It has a greater 

 number of tubercles in each row than the German species, in this respect resembling the 

 English form ; but, without the specimens were placed side by side, it would be impossible 

 to speak positively on the point of their entire identity. 



Pseudodiadema versipora is distinguished from Pseudodiadema mamillanum, which 

 occurs with it in the same bed, by the double row of pores in the upper part of the zones, 

 and by having secondary tubercles in the inter ambulacral areas; the bosses are likewise 

 smaller, and the tubercles are larger ; the miliary zones are much wider, and the component 

 granules are smaller. 



Pseudodiadema versipora resembles Pseudodiadema pentagona, M'Coy, in having the pores 

 in double pairs in the upper part of the zones ; but it differs form that Great Oolite Diadem, 

 which is a smaller, more depressed, and pentagonal form, in having larger primary and secon- 

 dary tubercles, with a wider, and more granular miliary zone, in the centro-sutural region. 



Pseudodiadema versipora resembles Pseudodiadema depressuni in size and form, but is 

 distinguished from it by having the pores in double pairs in the upper parts of the zones. 



