130 



CYPIIOSOMA 



tematic authors as specifically distinct ; and in deference to their opinion, rather than in 

 accordance with ray own convictions, I have devoted this article to its description. 



The test large and circular, inflated at the ambitus, convex above, and flat below 

 (PI. XXIII, figs. 2 a, b, c). 



The ambulacral areas are enlarged at the ambitus, moderately wide at the base, and 

 very narrow in the upper part ; by reason of the increased development of the poriferous 

 zones in this region, from the ambitus to the peristome, the tubercles are as large as 

 those in the inter-ambulacra, but in the upper third of the area they diminish rapidly 

 in size, and on the four or five coronal plates they are quite rudimentary (PI. XXIII, 

 fig. 2 a). The pores are arranged in oblique pairs, and are bigeminal from the ambitus 

 to the summit (PI. XXIII, fig. 2 c, (/), and unigeminal from the ambitus to the peristome, 

 the zones forming a series of crescents around the areolae of the ambulacral tubercles. 



The inter-ambulacral areas have two rows of primary tubercles, twelve to thirteen in 

 each row, which vary gradually in size from the ambitus to both apertures ; they are sur- 

 rounded by well-developed areola3, confluent at the upper and lower borders, and surrounded 

 by granules at the sides. The secondary tubercles are irregular, very small, and limited 

 to the under surface (fig. '2 b). On one or two plates above the ambitus there are only 

 one or two solitary tubercles, which are, however, inconstant in different specimens (fig. 2 d) . 

 The miliary zone is narrow and granular at the ambitus, and wide, depressed, and naked 

 on the upper surface ; the granules are unequal, of different sizes, and placed in semicir- 

 cular groups around the lateral parts of the areolae. Many of the granules are large and 

 mammillated, and are nearly as large as the row of secondary tubercles, the small gramdes 

 being compactly fitted in between them. The coronal plates are marked by slight sutural 

 impressions, which become more apparent in consequence of the nudity of the depressed 

 upper surface of the areas. 



The discal opening is very large, pentagonal, and angular ; and the shigle posterior 

 ovarial plate is projected far into the area (fig. 2 a). The mouth-opening is moderate in 

 diameter (fig. 2 b) ; the peristome is circular, and divided into ten unequal lobes by slight 

 incisions with reflexed borders. 



Afinities and Differences. — This species differs from the typical forms of Cyphosoma 

 Koniyi in the absence of a regular series of secondary tubercles above the ambitus ; but 

 in the general character of the test and in most of its details it has close affinities with 

 that species. Whether the spines of this Urchin exhibit any difference from those attached 

 to the typical form of C. Konigi (PI. XXIV, fig. 1) remains to be determined by those who 

 may be fortunate enough to discover a specimen with the spines adherent to the test. 



Locality and StratigrapUcal Position. — This Urchin is found in the Lower Chalk of 

 Kent and Sussex. The specimen figured in PI. XXIII, belonging to the British Museum, 

 was collected near Lewes. 



Foreign Localities. — Houguemarre(Eure),Orglande(Manche), from the Etage Senonien, 

 where it is rare (M. Cotteau) ; the type of Goldfuss's figure was collected from the Chalk 

 of Westphalia. 



