386 COMPARATIVE ANATOMY chap. 



and so is the stem, though the latter was present, since it is easy to distinguish a 

 depression at tlie apex marking its point of attachment. 



Eucystidea. 



Protocrinus (Fig. 260, p. 312). — The calyx is non-pedunculate, with sona-wliat 

 flattened apical side ; otherwise it is almost splierical. It con.sists of numerous 

 pentagonal or hexagonal bulging plates, each of which is provided with several 

 double pores. At the oral pole lies the oral aperture, from which five long anibu- 

 lacral furrows radiate, giving off', here and there, lateral furrows. At the end of each 

 lateral furrow there is a depression on a prominence. At these points, small arms 

 or pinnulae were perhaps once articulated. The ambulacral furrow and the mouth 

 are roofed over with covering plates. The anus lies excentrically in an interradius, 

 roofed over by a pyramid of valves. Between anus and mouth there is a small 

 third aperture. The stalked genus Ghjptosjjhcerites is related to Protocrinu.^. 



Orocystis (Fig. 261, p. 313). — The almost egg-shaped body is tessellated with a 

 considerable number of plates which are usually hexagonal, and are all provided 

 with pectinated rhombs. The pores are on prominences, arranged as in the figure. 

 A stem was present, but has never been met with attached to the body. On the 

 oral side of the body there are two princi2)al apertures, the mouth and anus, lying 

 on chimney-like prominences, and there is also an additional third aperture. The 

 area around the mouth is never preserved intact ; probably the mouth was 

 surrounded by a few tentacles. In the genus Pchinosphcera, very common in 

 the Lower Silurian, the spherical test is formed by a large number of pentagonal 

 or hexagonal plates, all of whieli have pectinated rhombs. In each rhomb the pores 

 lying on the opjiosite sides of the suture, between the two plates, are connected 

 in pairs by canals. The oral aperture lies on a chimney-like or conical promi- 

 nence, surrounded by two to four long or short arms. At some distance from the 

 buccal cone lies the anus, covered by a pyramid of valves. Between the mouth 

 and the anus, but a little to one side, is a third smaller apertui'e. In Aristocystis 

 there are two smaller apertures between the mouth and the anus, one of 

 which, near the anus, perhaps represents the genital aperture which, in other 

 Cystids, is possibly confluent with the anus. In Ascocystis, the body, which was 

 evidently richly plated, is prolonged like a tube ; at the pointed apical pole it was 

 attached by means of a stem ; at the oral side it is truncated, the oral disc being 

 surrounded by as many as twenty-five biserial, unbranched arms. The structure of 

 the disc surrounded by these arms has not yet been made out with certainty. 



Mesites. — The body is spherical, and was }irobably stalked. The test consists 

 of numerous plates provided with double pores, and showing no definite arrange- 

 ment. Five furrows run from the oral pole in meridians 

 towards the apical pole. Each furrow is covered by a double 

 row of contiguous, so-called ambulacral plates, and is thus 



converted into a closed canal. Between the consecutive (am- 



t^. , 'V\'t TrSiiis- 



.. ■ ,, „ „. bulacral) plates, pores lead into this canal, while on the plates 

 verse section through i ■ / • 



an amhulaorum of themselves circular areas can occasionally be made out, which 



Mesites. have been regarded as 2>oints of attachment of pinnidte. A 



groove runs along the middle line of each double row of plates ; 



this is open along most of its length, but, at the oral pole, small plates form a 



slanting roof over it. 



We thus distinguish, in each radius, an external groove which runs upon tlie 



ambulacral plates, and an internal canal, wliich runs below these plates and above 



the perforated plates of the test (Fig. 335). 



n] 



— <^ — 



