42 • CIDARIS 



Dimensions. — Height, nine tenths of an inch ; transvei'se diameter one inch and a half. 



Description. — This urchin, which forms so characteristic a species in the Lower Chalk 

 of the Continent, as shown in the table of synonyms, has hitlierto escaped the notice of 

 English geologists. The species from our Upper Chalk, en'oneously referred to this form, 

 being quite distinct from the true Cidaris vesiculosa, Goldf., from the Gray Chalk and 

 Upper Greensand ; specimens from each of these formations I have figured in Pis. II and III. 



The test is of moderate size, much inflated at the sides, and nearly equally depressed 

 at both poles ; the ambulacral areas are flexuous, depressed in the middle, and filled 

 throughout with numerous rows of small, regular, close-set, equal-sized granules ; at the 

 equator there are eight rows, which, near their terminations, diminish to two or four rows 

 (PI. Ill, fig. 1, d) ; the inner rows are those which gradually disappear, whilst the outer 

 rows are persistent throughout. 



The poriferous zones are very narrow, following the flexures of the ambulacra ; they 

 are composed of small round pores, separated by a granule, the series forming a moniliform 

 line down the middle of the zone, fig. 2. d. 



The inter-ambulacral areas are composed of very large plates, those in the upper half 

 of the columns being the largest and most inflated ; there are from four to five plates in 

 each series. The areas are circular and superficial, with very prominent borders sur- 

 rounded by a circle of distinct regular mammillated granules (PI. Ill, fig. 1, d) ; on the 

 under half of the test the tubercles are approximated (PI. Ill, fig. 1, b); whilst on the 

 upper half they are set wide apart, from the increased height of the plates (PI. Ill, fig. 1, 

 a and c) ; near the summit the uppermost tubercles on the alternate columns are imperfectly 

 developed, and often destitute of areolse, and placed in the middle of a long granular plate 

 (PI. Ill, fig. 1, a and c). The tubercles are moderate in size, and perforated ; the summits 

 of the bosses are quite smooth, and without any trace of crenulations (PI. Ill, fig. 1, e). 



The miliary zone is large and much depressed, and provided with numerous flat, 

 uniform granules, closely set together on the entire surface of the plates (fig. \, d). 



The peristome is circular (fig. 1 b), and the opening for the apical disc (fig. 1, a) is 

 nearly the same diameter as the oval aperture. 



The spines of this species, according to M. Cotteau, are elongated, cylindrical, sub- 

 fusiform, with compressed longitudinal ribs, more or less spinous, and denticulated, the 

 processes being always at regular distances apart. Towards the base, the ribs diminish 

 and disappear, and the stem is provided only with fine, close-set striae. The neck is short 

 and striated, the milled ring prominent, the articular head smooth, and surrounded by a 

 small groove. 



Length, twenty-five to thirty millimeters ; thickness, five millimeters. 



Variety, sj)inulosas, Agassiz : length, forty-seven millimeters ; thickness, six millimeters. 



Affinities and differences. — This species is distinguished from Cidaris sceptri/era, 



