384 CLYPEUS 



The single inter-ambulacrum forms one of the most remarkable features in the 

 anatomy of the test ; the anal valley is short, deep, and limited to the lower third 

 of the dorsal portion of this area (fig. 1 a) ; the posterior part of this segment is 

 rostrated, and much deflected (fig. 1 d), and its extremity abruptly truncated (fig. 1 a, d) ; 

 the arch over the vent is narrow above, its sides slope a little inwards, and the opening 

 expands towards the border (fig. 1, a, d) i the vent opens near the surface beneath the 

 arch (fig. 1 d) ; it has an oval form with its long diameter towards the vertex. 



The base is concave (fig. 1 d), and the inter-ambulacra form prominent cushions 

 between the narrow ambulacra, the convexity of these segments produces a considerable 

 undulation in the border, which is well represented in fig. 1 c, d ; the mouth-opening is 

 sub-central, nearer the anterior than the posterior border, it has a pentagonal form, and 

 the peristome is surrounded by five prominent oral lobes (fig. 1 d). 



The apical disc is very small, it occupies the centre of the upper surface, and forms 

 the vertex of the test; I have only seen one specimen in which this part is preserved. 

 The separate plates are so intimately soldered together, that I can only recognise the four 

 genital holes M'hich perforate the test obliquely (fig. 1 y), and the five ocular holes at the 

 summits of the ambulacra ; the surface of the discal plates is covered by a spongy 

 madreporiform body which forms the most conspicuous element in the small central disc 



(%. ly)- 



Affinities and differences. — Clypeus suhulatus is frequently mistaken for Fygurus 

 pentac/onalis, Phil., and lies with this name attached, in several Yorkshire collections. In 

 fact, the oblong figure of the test, the apparent absence of an anal valley, which is always 

 filled up with matrix, and seldom cleared out, the form of the ambulacral petals, and the 

 central position of the small apical disc, produce an assemblage of characters which have 

 misled local observers. M. Desor, in his synopsis, has placed this Clypeus in the genus 

 Pygurus, and has added this note to his diagnosis — " Cette position exceptionnelle du 

 pcriprocte n'est pas une raison suffisante pour eloigner cette espece au Pygurus auxquels 

 elle correspond par tours ses autres caracteres." I can readily understand how this 

 mistake has been committed, if M. Desor's opinion was formed from Phillips's figure, 

 the only one up to the present time worthy of the name of a sketch which has been 

 published. 1 trust, however, that Mr. Bones's beautiful plate, with its ample details 

 will set the question at rest, for there can be no doubt that this urchin is a 

 true Clypeus. It has no affinity with any other English congener ; its concave under 

 surface, and strongly-cushioned basal inter-ambulacra, resemble the base of Clypeus 

 alius ; but that species has an orbicular outline, and a long, narrow, anal valley, which 

 widely separate it from Clypeus suhulatus. It more nearly resembles Clypeus Rathieri, 

 Cott., from the Eorest marble of Chatel-Gerard (Yonne), than any other species ; but the 

 narrowness of the anterior border, the flatness of the base, and the excentral position of 

 the disc, prove how specifically distinct they are from each other. I may add, that the 



