50 CIDARIS 



The inter-ambiilacral areas are wide (PI. IV, fig. 1 ; PI. V, fig. 2, a, b), Avith five very 

 large, prominent tubercles in each row; the areolas are narrow, deep, and circular, closely 

 approximated near the peristome (PI. V, fig. 2, b), and wide apart at the upper surface 

 (PI. V, fig. 2, a) ; the margin is surrounded by a circle of small mammillated granules 

 (PI. IV", fig. 1 , b, c), a little larger than those which fill the miliary zone ; the boss is 

 small, and not prominent, and its summit quite smooth (PI. IV, fig. b, c) ; the tubercle 

 is very large and prominent (PI. V, fig. 2 ; and PI. IV, fig. \,b, c);\i forms three fourths 

 of a sphere, and the surface is imperforate ; the tubercle nearest the disc is sometimes 

 surrounded by a rudimentary areola (PI. IV, fig. 1, a ; PI. V, fig. 2, a), and placed 

 wide apart from its penultimate fellow. The areolas at the under surface are so closely 

 approximated that many of them are separated only by a single row of granules. 



The apical disc (PI. V, fig. 3) is wider in diameter than the peristome ; the five ocular 

 plates are large, with prominent orbits surrounded by a radiated structure; the ovarial 

 plates have a rhomboidal figure, with the hole distant from the border ; the anal plates 

 are well preserved in the specimen fig. 3, in which sixteen plates remain i7i situ; the 

 surface of all the elements of the disc is covered with small granules, set well apart from 

 each other; fig. 3 shows the disc and its elements magnified two and a half times. 



The jaws and teeth are in situ in the specimen belonging to the British Museum 

 (PI. V, fig. 1) ; the peristome is small (fig. 2, b) ; and more than half the opening was 

 filled by a buccal membrane, on which two or three rows of scale-like plates are disposed 

 in an imbricated manner. PI. V, fig. 4, exhibits the buccal membrane and its plates 

 magnified two and a half times. 



The form of the spines varies in different parts of the test. In general they are 

 claviform, more or less developed, with massive bodies, elongated necks, and small 

 articular heads. The body and neck are ornamented with numerous longitudinal ridges ; 

 those on the massive body develope a denticulated, spiny margin, as seen in PL IV, fig. 5, 

 and in other series of spines figured in this jjlate. The denticulated ridges are sometimes 

 prominent and regular, unequal, oblique, and rudimentary ; on the neck the ridges are in 

 general smaller, longitudinal, and with finer denticulations on the margin ; at the inferior 

 third of the neck they diminish in size and disappear, and are replaced by granules, which 

 likewise fill up all the intermediate space between the ridges, both on the neck and 

 massive head. In some spines, with a large head and short neck, the weight of the 

 spine is diminished by a series of canals, which extend into the head beneath the spaces 

 between the ridges; the head is sometimes traversed by a large channel, opening at the 

 sumn)it, as in a specimen (PI. V, fig. G) from the Eev. T. Wiltshire's collection. The 

 collar is very small, and finely striated ; the milled ring is narrow, and separated from the 

 collar by a depression ; both ring and depression being finely milled ; the acetabulum (fig. 

 2) is moderately deep, with a well-defined ring round the margin. In Plate IV, I have 

 figured the finest specimen extant of this species ; it belongs to my friend Dr. Bowerbank, 

 and shews two tests with their spines attached in situ. This specimen exhibits the various 



