294 CYSTOIDEA, BLASTOIDEA, AND HOLOTHUROIDEA. 



In another great group of Cystidea, lastly, we have genera 

 in which the plates of the test are definite in number, and 

 more or less clearly arranged in a quinary manner, while 

 the respiratory pores are usually confined to special and 

 limited areas of the perisome (" pectinated rhombs "). The 

 number of types included in this group is very large, and 

 only some of the most important can be noticed here. One 

 of these namely, the Pleurocystites of Billings (fig. 172, B) 

 is a transitional link between the present and the preceding- 

 groups ; since the perisomatic plates of the dorsal side of the 

 calyx are large and definitely arranged, while those of the 

 ventral side are small, numerous, and indefinite. There are 

 two free, jointed pinnulse, or unbranched arms, and there 

 are three " pectinated rhombs." The genus is wholly Lower 

 Silurian. A more typical member of this group is Echinoen- 

 crinus (fig. 174, c and D), in which, as in various allied 

 genera, the calyx is composed of four series of plates, there 

 being four basals and five plates in every range above this. 

 There are three " pectinated rhombs," and, so far as known, 

 no arms. The genus is found in the Lower and Upper 

 Silurian. Glyptocystites, of the Lower Silurian, is a near 

 ally of the preceding, but possesses numerous "pectinated 

 rhombs " (ten to twelve in number), no other known genus 

 having more than three of these organs. Apiocystites, Pseudo- 

 crimes, and Callocystites, form a closely-related sub-group, dif- 

 fering from G-lyptocystites in various points, but agreeing with 

 it in possessing five recumbent arms, which are soldered to 

 the calyx, on the surface, in the form of so many grooves, 

 bordered on each side (when perfect) with a row of short 

 jointed pinnulae. In Pseudocrinus (fig. 172, c) the calyx is 

 orbicular, and the arms, though recumbent, project consider- 

 ably above the surface; in Apiocystites (fig. 175, E) the body 

 is oblong and tetragonal ; and in Callocystites the body is 

 ovoid (fig. 175, D), and the arms are deeply furrowed. 

 These three genera are characteristic of the Upper Silurian 

 period, as are Lepadocrinus (fig. 172, D) and Prunocystites ; 

 the former of these being peculiar in having the last joint 

 of the column very much elongated and pointed, while the 



