ECHINODERMATA. CYSTIDEA. 93 



radiate food-grooves (6), this may therefore be regarded as 

 the mouth. On the upper surface also there is another 

 opening regarded as the anus (d), it is covered with plates 

 which form a cone. In some forms, near the mouth there 

 is a smaller aperture, which may be a genital pore (e). 

 A characteristic feature of the plates of the calyx is the 

 presence of pores which pass right through the plates. 

 In a few forms they are absent. They may either be 

 arranged in pairs (fig. 23 B) placed in oval depressions or 

 on elevations ; in other cases they form what are known 

 as pore-rhombs (fig. 23 (7), these consist of rhombic figures, 

 the outline of which is formed by the pores, half the 

 rhomb being on one plate, and half on the adjoining, and 

 the pores of the two plates are united by grooves. The 

 number of these pore-rhombs varies considerably in 

 different genera. 



The cystideans then resemble the crinoids in the 

 general form of the calyx and stem, in the position of the 

 mouth and anus, and in the possession of food-grooves 

 and arms. But they differ in having the arms feebly 

 developed, in the irregular arrangement of the plates and 

 in the presence of pores. 



Lepadocrinus (= Pseudocrinites). Calyx ovate, rhom- 

 bic in outline, with rounded edges ; at the base is a cycle 

 of four plates followed by three cycles of five plates each. 

 Mouth central. Two to four straight food-grooves, ex- 

 tending to about the middle of the calyx, and with 

 projecting edges. Anal aperture near the summit, covered 

 with six triangular plates. Three pore-rhombs, one near 

 the base, two on the upper surface of the calyx. Stem 

 thick, circular, rather long, becoming thinner towards the 

 lower extremity. Silurian. 



