138 CYPHOSOMA 



areas are wide; with two rows of tubercles, which are large at the ambitus and base 

 and veiy small on the upper surface ; the change from the large ambital to the small 

 dorsal tubercles is very abrupt ; the areolae of all the large tubercles are wide and 

 confluent, and those of the smaller are surrounded by. circlets of granules. 



The poriferous zones are narrow, and much undulated at the ambitus and infra-mar- 

 ginal region ; they are less sinuous above, where the pores are small and unigeminal ; the 

 spaces between the pairs of holes often support two small granules (fig. 1 e). 



The inter-ambulacral areas are about one third wider than the arabulacral, and pro- 

 vided with two rows of primary tubercles, twelve in each, nearly identical with those in 

 the ambulacra; like them, they have wide areolae at the ambitus and infra-marginal region, 

 and very small on the upper surface ; the transition from the large ambital to the small dorsal 

 tubercles is likewise well marked in the specimen before me (fig. I a, d). A row of small 

 irregular secondary tubercles, five or six in each, occupies the spaces between the primary 

 series and the poriferous zones (figs. 1 d, d), and disappears among the granules of this 

 region. Other secondary tubercles occupy the wide spaces by the zones at the angles of the 

 plates (figs. 1 c, e, and figs. 2 a, b), and range in file with the small secondary series on the 

 upper surface. The intermediate granules are large and abundant, of unequal size, and 

 disposed in circles around the areolae ; these circles at the ambitus are incomplete at 

 their basal border (fig. 1 e). The miliary zone is large, much depressed in the middle, 

 and nude as it approaches the discal opening (fig. 1 d) ; the small granules forming complete 

 circlets around the small tubercles of this region. 



The primary tubercles at the under surface are moderately large, and nearly the same 

 size in both areas (fig. 1 c, and fig. 2 b), which imparts a highly ornamented character 

 to this region of the test and contrasts strongly with the small tubercles, and the naked 

 and depressed miliary zone on the upper surface (fig. 1 d). 



The mouth-opening is small, the peristome circular, and divided into nearly equal- 

 sized lobes by feeble incisicms (fig. 1 c). The discal opening is large, pentagonal, angular, 

 and elongated in the antero-posterior direction (fig. \ b). 



C^hosoma vnic/nijicum, so rare in England, is a very common species in the south- 

 west of France, Avhere two well-marked varieties are found. The/r*/f type of the species 

 is characterised by having its ambulacral and inter-ambulacral tubercles large and promi- 

 nent at the ambitus, becoming gradually smaller on the upper surface, the miliary zone wide 

 and not depressed in the middle. In the second type the transition in size from the large 

 ambital to the small dorsal tubercles is more abrupt, the upper part of the miliary zone is 

 quite destitute of granules and much depressed in the middle, near the coronal plates, which 

 are marked with very distinct sutures ; the areolae at the ambitus are larger and more super- 

 ficial, and the under surface has a more ornamented appearance. This variety has been 

 described as C. sulcatum, and is that to which our specimen is referred. It attains a much 

 larger size than the first or type form, as a specimen collected from the Chalk of Royan 



