CYSTOIDEA. 133 



attained their maximum of development in the Lower Silurian 

 period, to have gradually diminished in numbers and in im- 

 portance during the Upper Silurian, and to have died out 

 at the commencement of the Devonian period, or shortly 

 thereafter. 



Of the commoner genera, Hemicosmites (fig. 82), Caryocrinus, 

 and Echinospheerites have all the plates of the calyx perforated 

 by numerous pores. Spharonites has each calycine plate per- 

 forated by two pores, and Apiocystites, Callocystites, Prunocys- 

 tites, and Echinoencrinus have the calycine pores arranged in 

 rhombs upon two or three of the plates of the calyx. In 

 Glyptocystites there are from ten to thirteen of these " pecti- 

 nated rhombs." In Malocystites, Apiocystites, Callocystites, and 

 Psetidocrinus the arms are recumbent and soldered to the 

 calyx. In Comarocystites there were four free arms with pin- 

 nulae. In Lepadocrinus the last joint of the column is very 

 much elongated, and pointed at its extremity ; and in Agela- 

 crinites and some other forms the calyx appears to have been 

 directly attached to some foreign body, and a column seems 

 to have been wanting. 



ORDER VI. BLASTOIDEA. 



The Blastoidea or " Pentremites " are Echinodermata in 

 which the body is enclosed in an armour of closely-fitting calcareous 

 plates, and is fixed to some foreign object by a slender column. 

 From the summit of the calyx radiate five areas which are grooved 

 longitudinally, and striated across, and which carry a row of 

 jointed pinnula on each side, (Fig. 85.) 



The Blastoidea nearly resembled the Cystideans in many 

 respects. The body or "calyx" is enclosed in a series of 

 articulated calcareous plates consisting of three large basals, 

 carrying five forked plates, which are usually regarded as 

 corresponding with the " radials " of the Crinoids. At the 

 angles of junction of the forked "radial" plates rest five 

 smaller plates the so-called " deltoid plates " which con- 

 verge towards the centre of the summit of the calyx. Between 

 these deltoid plates which are generally regarded as corre- 

 sponding with the " inter-radials " of the Crinoids are the so- 

 called " pseud -ambulacra." The pseud - ambulacra radiate 

 from the summit of the calyx, and the apex of each is re- 

 ceived into the cleft formed by the bifurcation of one of the 

 "radial" plates. Each pseud-ambulacrum is furrowed by a 

 longitudinal groove down its centre, and is of a somewhat 

 petaloid shape, the areas on each side of the central groove 



