130 PSEUDODIADEMA. 



the genital plates are of a pentagonal form (fig. 1 e) ; the anterior pair are the largest, and 

 the right antero-lateral, supporting the madreporiform body, is much larger than the left 

 antero-lateral ; the postero-lateral and single plates are about the same size ; the genital 

 holes are small, and pierced near the external prominent angle of the plates ; the ocular 

 plates are small, heart-shaped bodies, placed at the summits of the ambulacra, which they 

 terminate ; the eye hole is marginal, and proportionally large ; the anal opening is oblong, 

 its longest diameter extending obliquely across the test ; the surface of all the elements of 

 the apical disc is covered with small, nearly equal-sized granules. 



The base is flat (fig. 1 c), the mouth opening is large, being more than one half the 

 diameter of the base ; the peristome is decagonal, and divided into ten lobes by deep 

 notches, the ambulacral being much larger than the inter-ambulacral lobes (fig. 16). 



The spines are not preserved in any English specimens I have seen. Those figured 

 in situ, by Agassiz, are long and needle-shaped, and their surface is covered with fine 

 longitudinal lines. 



Affinities and differences. — Pseudodiadema hemisphcericum difiers so much from all our 

 other English species of this genus, that it is impossible to mistake it for either of them. 

 Some juvenile specimens, however, have an air of resemblance to Hemicidaris pustulosa, 

 arising from the disproportionate size between the tubercles at the base and equator of 

 the ambulacra, and those occupying the upper portion of these regions ; but the presence 

 of secondary tubercles in the inter-ambulacra, flanking both sides of the primary tubercles, 

 serves as a diagnostic character, no Hemicidaris having any secondary tubercles in the 

 inter-ambulacral regions ; moreover, the number of the primary tubercles in each row is 

 considerably more in Pseudodiadema hemispheericum than in Hemicidaris pustulosa. 



Lamarck's species is distinguished, according to M. Cotteau, from the urchin 

 figured by him under the name Diadema hemispharicum, "d'une maniere positive 

 et constante, par ses tubercles secondaire plus developpes, plus nombreaux, et plus regu- 

 lierement disposes au milieu des aires inter-ambulacraires ; il s'en distingue egalement par 

 les entailles plus profondes de son ouverture buccale."* 



Pseudodiadema hemisphisricum, Agass., resembles Pseudodiadema Orhignyanum,, Cotteau, 

 but the latter is readily distinguished from it by the greater number, regularity, and 

 uniformity of its secondary tubercles, and the smaller size of the primary tubercles. These 

 three species, which are all characteristic of the Coral Rag, resemble each other very 

 much, and are only distinguished by the number and disposition of their secondary 

 tubercles, which are rare in Pseudodiadema hemispJiaricum, Cotteau, more numerous 

 in Pseudodiadema hemispJiaricum, Agass., and very abundant in Pseudodiadema 

 Orhignyanum, Cotteau. 



Locality and StratiyrapMcal position. — This species is very rare in the Coralline 

 * 'Etudes sur les Echiiiides Fossiles,' p. 144. 



