THE CYSTIDEA 67 



imagine the anterior radius to be doubled, concurrently with the 

 bifurcation of the anterior arm, and may therefore speak of the left and 

 right anterior RR and IBB and the anterior B, while the other plates 

 retain their usual names. The three iRR rest on the truncated upper 

 margins of the anterior and right and left antero-lateral BB, and are half 

 the width of the RR, but quite as high. The anus, with valvular 

 pyramid, is left of the posterior interradius, being in fact below L post. 

 R, and between 1. post, and post. BB. The tegrnen is solidly covered 

 with nine plates alternating with the nine plates of the radial circlet 

 (Fig. XXXVI. 1). These plates have no pores like those of the dorsal 

 cup (but the posterior one is rugose at its adoral end, and appears to have 

 been pierced by a hydropore). From the central mouth three food- 

 grooves run over these plates towards the anterior and the right and left 

 antero-lateral RR. The mouth and grooves are protected by relatively 

 large irregular covering-plates. At the end of each groove is an oval 

 area over which passed the base of a biserial arm, which became free almost 

 immediately. Caryocrinus, Say (1825, see von Buch, 1845, and Hall, 1852 ; 

 synn. Stribalocystis, S. A. Miller; Enneacystis, Haeckel), Upper Ordovician, 

 Scandinavia, and Silurian, N. America. Dorsal cup (Fig. XXXVI. 4) differs 

 from that of Hemicosmites only in absence of anterior iR, and of anus, the 

 latter having moved up to the tegmen. The covering-plates of the food- 

 grooves have become larger and incorporated in the tegmen (Fig. XXXVI. 2), 

 so that the grooves are subtegminal (cf. Crinoidea Camerata) ; but 

 end in facets on the margins of the RR and iRR. Since the grooves 

 branch while beneath the tegmen, these facets are more than three, though 

 still distributed into a left, right, and anterior group. In the growth of 

 the individual, Hall has noticed successive stages with from three to 

 fourteen facets ; six and nine are fairly common, but for adults thirteen 

 is the usual number, the thirteenth being added on the right of the anus. 

 Hall describes the arms as composed proximally of "semicircular, and 

 scarcely interlocking " ossicles ; distally the brachials alternate so that 

 the arm is biserial. Each brachial bears a grooved pinnule, perhaps also 

 biserial. In Corylocrinus and Juglandocrinus (both von Koenen, 1886), 

 Upper Ordovician, South France, the composition of the dorsal cup (Fig. 

 XXXVI. 6) is the same as in Caryocrinus. The tegmen of Corylocrinus 

 is composed of four plates, of which the right, left, and anterior bear arm 

 facets, apparently as in Hemicosmites ; the posterior tegminal plate is very 

 thick and porous (a madreporite). In Juglandocrinus (Fig. XXXVI. 5), as 

 in a young Caryocrinus, the food-grooves are subtegminal, and come to the 

 surface on the upper margins of three large plates, which are right, left, 

 and anterior in position, and alternate with 'three smaller plates. These 

 six plates exactly correspond to the tegmiual plates of Hemicosmites; a 

 central plate over the mouth, and three plates covering the food-grooves, 

 represent the covering-plates of that genus. This interpretation of von 

 Koenen's obscure genera was partly suggested by P. H. Carpenter (1891). 

 An anus was doubtless present, though not observed in the imperfect 

 specimens. Heterocystis, Hall (1852), Silurian, New York (Fig. XXXVI. 7), 

 appears to have been derived from Hemicosmites by the vertical bisection of 

 4 BB (viz. r.post., r. ant.-lat, ant., 1. ant.-lat.), thus producing 10 BB ; 



