FROM THE WHITE CHALK. 59 



marginal region and near the peristome ; the margin is surrounded by a complete circle of 

 large mammillated granules raised on sliield-like plates. On the two uppermost plates of 

 each column the areolae are very narrow, or altogether obsolete (fig. 4, a). The boss has a 

 flat smooth summit (fig. 1, c), and the large tubercle is deeply perforated (fig. 1, c and d). 

 The miliary zone is very wide at the equator, and becomes narrower near the peristome 

 and the disc. It is more or less depressed in the middle, along the line of the sutures, 

 and covered with fine homogeneous granules set closely together; the granules are 

 arranged in regular horizontal lines (fig. 1, c, d, e), which radiate from the circumference 

 of the areolae to the border of the plate. 



The apical disc is large and pentagonal (fig. 4, a, b), and well preserved in situ in the 

 fine specimen, fig. 4, a. The five large ovarial plates have an irregular rhomboidal form, 

 with the ductal holes near the border ; the oculars are heart-shaped, and have marginal 

 orbits ; the anal plates form a double series within the discal circle, and the vent (fig. 4, b) 

 is a small e.xcentral aperture with a third series of small plates on its anterior part only. 

 All the elements of the disc are closely covered with the same style of grannies that fill 

 the miliary zone. 



The peristome, smaller than the discal opening, is of a pentagonal form ; in none of 

 our specimens are the dental pyramids preserved. 



The spines are long, slender, cylindrical, with longitudinal ribs having a spinous 

 border (fig. 2, a, b) ; the valleys between the elevations have a finely chagreened surface ; 

 the neck is short and striated, the head moderately large, and the milled ring prominent 

 (fig. 2, c). The acetabulum has a smooth rim. One spine must have measured 2^ inches 

 in length. The large mammillated granules surrounding the areolae supported small, fiat, 

 triangular spines (fig. 6), having their surface ornamented with longitudinal microscopic 

 lines, and articulated to the tubercle by a semicircular depression at the base. I have 

 figured one of these scrobicular spines at fig. 6, where the line shows the natural size, and 

 the figure is enlarged four diameters. 



Affinities and differences. — This species has long been considered to be the Cidaris 

 vesiculosa, Goldf., but is distinguished from that urchin in having the ambulacra less 

 flexed, a greater number of plates in the inter-ambulacral colnmns, the upper tubercles' 

 of both series rudimentary, and in having the granvdes in the miliary zone arranged in 

 horizontal rows. Cidaris vesiculosa, Goldf., has a smaller test, the ambulacra much more 

 flexed, the plates in a column fewer, the tubercles consequently wider apart, the upper 

 tubercles largely developed, and the grannies in the miliary zone not arranged in hori- 

 zontal rows. The spines in C. snbvesiculosa are long, slender, and tapering ; those in C. 

 vesiculosa are shorter and thicker. This urchin so closely resembles C. perlafa, Sorignet, 

 and C. Vcndocinensis, Ag., that they appear to me to be only varieties of C. subvesiculosa, 

 d'Orbig. ; the spines of the latter likewise closely resemble those attributed to C. serrata, 

 Desor. It is possible that if a collection of good type-specimens of these different reputed 



