VIII ECHINODERMATA— MORPHOLOGY OF SKELETON 385 



of a rhomb are not infrequently divided by a smooth intei'mediate 

 region. 



Since it is difficult or even impossible to give any comprehensive description of 

 the perisomatic skeleton of the Cystidea, it is advisable to treat a few of the best 

 known forms separately. 



Cystocrinoidea. 



Porocrinus is a form which only dilTers essentially from a simple Crinoid of the 

 order Inadunafix in the presence of the pectinated rhombs. 



Caryocrinus. — The calyx is almost pear-shaped, and is carried by a long stalk 

 perforated by a wide axial canal. At the edge of the calyx there are 6 to 13 

 thin, jointed, biserial arms, each provided with a furrow on the oral side. The 

 hexagonal dorsal cuji, apart from two plates lying interradially in the circle 

 of the radials, consists exclusively of the already described plates of the apical 

 system (four infrabasals, six basals, six radials). The tegmen calycis is formed of a 

 great number of plates, and at their centre six orals, which show the characteristic 

 arrangement before described (Fig. 294, p. 332), completely cover the mouth. The 

 anibulacral furrows are not externally visible. An excentrically placed aperture, roofed 

 over by a pyramid formed of six triangular plates, is regarded as the anus. The 

 pectinated rhombs are found on all the plates of the dorsal cup, and on them alone. 

 The two pores of a double pore are connected by a canal on the inner side of the plate. 



Echinoencrinus. — The almost egg-shaped calyx consists of the already described 

 plates of the apical system (four infrabasals, five basals, five radials) arranged in five 

 rays, and five other perisomatic plates in contact with the circle of the radials, and 

 partly pressing in between them. At the oral pole there is a depression, round which 

 short uniserial tentacles rise, and in the centre of which the star -shaped oral 

 aperture lies. The anal aperture has shifted across the equator of the calyx 

 apically, and lies to the right posteriorly above the basals. Three pectinated 

 rhombs are developed, whose position is shoAvn in Fig. 296, p. 333. The calyx is 

 carried by a short thick stem, the ossicles of which are perforated by a wide canal. 



Cystoblastus (Figs. 259, p. 312, and 295, p. 332) shows some resemblance to 

 the Blastoidea. The egg-shaped or spherical calyx consists of sixteen plates and the 

 ambulacra. Of these sixteen plates, which are arranged in four circles, fourteen 

 belong to the apical system (four infrabasals, five basals, and five radials). The 

 radials exactly resemble the radials (fork plates) of the Blastoidea. Each radial 

 clasps an ambulacrum between its two limbs. Between two neighbouring radials, 

 in four of the interradii, a lancet-shaped plate, which is keeled in the middle, 

 presses in, recalling the deltoid plates of the Blastoidea. In one interradius this 

 [ilate is wanting, so that here the neighbouring radials touch one another. At the 

 centre of the rosette formed by the five ambulacra lies the mouth, and from it a 

 furrow runs to each ambulacrum, passing down its whole length and dividing it into 

 two latei'al halves. The principal groove of each ambulacrum gives ott' alternating 

 lateral furrows, which end in distinct pits (pores ? depressions for the reception of 

 pinnulte ?). At the base of the calyx there are two pectinated rhombs {cf. Fig. 

 295, p. 332). Further, the forks of the radials aj^pear transversely striated by 

 numerous parallel pore clefts, and a similar striation also occurs on each side of the 

 rib or keel on the four deltoid plates. Perhaps every two neighbouring rows of pore 

 clefts, belonging, however, to diff"erent but adjoining plates, together formed a kind 

 of pectinated rhomb. A large aperture half way uj) the calyx is regarded as the 

 anal aperture, and a smaller, in an angle between two ambulacra, as the ajierture of 

 the water vascular system. This is, however, uncertain. The arms are unknown, 

 VOL. II 2 c 



