STRUCTURE OP THE CYSTIDE^. 41 



oral pinnulae : Prunocystitcs — and 3rd, armless forms : Caryocystites 

 and SplicRronites ; to which latter the British form Echino-encrinus 

 is added. Forbes considers that the arms observed by Volborth 

 in the Russian species of Eddno-cncrinus are oral pinnulae. The 

 oral arms of Echino-encrinus and Prunocysdties are articulated in two 

 series. Volborth observed that in the former they are beset with 

 small plates upon their ambulacral surfaces, which he calls 

 tentacles, remarking that pinnulas are absent. These plates have 

 the characters of marginal plates, which in the Crinoids {Pentacrinus) 

 occur on the arms as well as on the pinnulae. In Echino-encrinm 

 angulosus the remains of six arms were present. That this number 

 does not agree with the five depressions which usually surround 

 the mouth is explained by the fact, that the number of these 

 facets varies ; Von Buch states that there are five or six ; and I 

 possess a specimen with eight round depressions about the mouth, 

 which are united with the mouth by grooves. Echino-encrinus 

 striatus possesses, according to Volborth, together with a very 

 much narrower pointed oral extremity of the calyx, only two 

 much larger opposed oral arms, which have the same structure 

 as in Echino-eticrinus angulosus. From their relations, however, 

 it is probable that these are not pinnuloe, but arms ; for it is not 

 usual for pinnulae to be isolated. If they both belong to a single 

 ambulacrum how are we to imagine a single ambulacrum in this 

 locality in the immediate neighbourhood of the mouth ? If, 

 however, they belong to two different ambulacra, they can, as 

 solitary structures, be only arms. 



"The arms of EchinosphtBrites aurantiwn, Wahlenb. {Sphceronites 

 aurantium, His.) have essentially exactly the same relations as 

 Volborth has described and figured. In such well-preserved 

 specimens as now lie before me, the origins of three articulated 

 arms at the oral region of the calyx are recognizable. The five 

 uppermost calycine plates are raised into a three-sided pyramid 

 transversely truncated above, whose obtuse edges are prolonged 

 into the arms. Two sides of the pyramid are broader than the 

 third. The sutures between the five pieces are so disposed that 

 two of them are situated upon the broader side of the pyramid, 

 the three others in the obtuse edges. Two supplementary pieces, 

 however, are added to the five principal portions of the pyramid, 

 and extend from the calyx into two of the angular sutures. The 

 pore-grooves of the plates of the calyx extend only on to the 

 lower portion of the circumference of all the seven pieces. The 



