BLASTOIDEA. 457 



below, or ends in an elongated and pointed ossicle (fig. 326, d). 

 The mouth is central, and there are two or four ambulacral grooves, 

 which in well-preserved examples are bordered by short pinnules 

 (fig. 326, c). The anus is excentric, and is furnished with valvular 

 plates, and there are three reniform or triangular pore-rhombs. The 

 genus is principally Silurian. Lastly, in Cailocystites, as in the pre- 

 ceding genus, the calyx consists of four zones of plates, of which the 

 lowest consists of four basals. The mouth is central, and there are 

 five ambulacral grooves (fig. 320), which are bordered by well-devel- 

 oped pinnules. The anus is excentric, and there are four pectinated 

 rhombs. The species of Cailocystites are found in the Silurian rocks 

 of North America. 



Class III. Blastoidea. 



This class includes extinct Echinoderms in which the body- 

 ("calyx") is pyriform, clavate, ovate, or globular, and is attached by 

 a short jointed peduncle, zuhich is in some cases wanting. The calyx 

 exhibits complete radial symmetry, and is enclosed in a covering of 

 suturally united calcareous plates, consisting of basals, radials, and 

 interradials. The upper surface exhibits five ambulacral areas, 

 usually of a more or less petaloid shape, which radiate from the 

 mouth, the latter being superior a?td central, and being concealed by a 

 covering of small calcareous plates, some of which represent the " orals " 

 of the Crinoids, while the ambulacral grooves are protected by a double 

 row of '" covering-plates" Small jointed appendages or "pinnules" 

 are attached, in a single or double row, to the sides of the ambulacral 

 fields, but no true arms are present. " Hydrospires," opening by jive 

 or ten apertures round the mouth or by interradial fissures, are present, 

 but are limited to the radial and interradial plates, never extending on 

 to the basals. 



The Blastoids form a peculiar group of Palaeozoic Echinoderms, 

 in which the body is generally supported by means of a short 

 peduncle composed of discoidal joints, though in some cases no 

 stem appears to have been present. The calyx is globular, ovoid, 

 clavate, or bell-shaped, and is remarkable for its regular radial sym- 

 metry. Underbasals, such as are found in the " dicyclic " Crinoids, 

 are not developed, but the calycine plates consist of a cycle of 

 basals, a row of radials, and a circle of interradial (" deltoid ") plates. 

 The basals (fig. 327, a and 328, b) are three in number, two 

 being comparatively large and of equal size, while the third is much 

 smaller. Following the basals is a cycle of five "radials," which 

 are usually of large size, and form the sides of the calyx (figs. 327 

 and 328, d). The radials have been sometimes spoken of as the 

 " forked plates," owing to the fact that each is deeply divided or 



