54 



THE CYSTIDEA 



(cf. p. 11). In either of these cases the anterior arm may diminish in size 

 and finally disappear, leaving either two lateral, or two antero-lateral + two 

 postero-lateral = four branches in all. The length of the arms and their 

 subsequent branching, if any, are unknown. The portions preserved are 

 formed of series of brachials, bearing on their adoral (upper) surfaces a 

 groove leading to the central mouth, and roofed by small covering-plates. 

 The anus (As), with its pyramid of four to ten plates, lies 1/3 or 1/4 of the 

 way down the theca. Between anus and mouth, to the right, is the hydro- 

 pore (M). The plates of the theca were united by strands of mesostroma as 

 in the next genus. Arachnocystis, Neumayr (1889), of Ordovician age, 

 has for type the Echinospliaerites infaustus of Barrande (1887). The 

 pear-shaped theca is composed of 200-800 plates of irregular shape and 

 arrangement, mostly small, but with a few larger plates interspersed. At 

 the narrower end of the theca is a stem, about 40 mm. long, composed 

 of five (?) alternating rows of hexagonal plates. At the opposite pole lies 

 the mouth, on a slight elevation of larger, irregular plates (not five orals 

 as sometimes stated). From these are given off three arms, composed of 

 two alternating rows of plates (biserial), and with a ventral groove roofed 

 by small covering-plates ; they may reach 100 mm. in length. About a 

 third of the way down the theca is the large anal opening, closed by a 

 pyramid of five plates. The hydropore has not been observed. The 

 structure of the thecal plates is clearly shown in the Bohemian fossils ; 

 between the thin non-porous epistereom and hypostereom lies the ineso- 

 stereom, penetrated by canals left by the strands of mesostroma that ran 

 at right angles across the sutures and united the plates. A trace of the 

 original path of these strands, as seen in Aristocystis, remains in the form 



of canals passing down to the hypostereom, 

 one at the ad-central end of each transverse 

 canal, and one on either side the suture. 

 The centre of each plate is solid, and often 

 raised in an umbo ; by ideal lines drawn 

 from the umbo to the angles of each plate, 

 the canals are grouped in triangles, and 

 the adjacent triangles of two neighbouring 

 plates form a " pore-rhomb." Palaeocystis, 

 Billings (1858), Ordovician, Canada, has 

 thecal plates of similar structure. Orocystis, 

 Barrande (1887), Ordovician, Bohemia 

 (Fig. XVI.), has an oviform theca, with a 

 small hexagonal stem (St) of unknown 

 length, and near the other pole three 

 eccentric openings : generally, and 

 probably with right, regarded as the 

 restored on mouth ; Ait the anus ; M the hydropore. 

 Th * thecal plates are marked with strong 

 axial folds, parallel with which are smaller 

 ridges, all at right angles to the sutures. The folds are probably the 

 superficial indications of axial nerves, and it is noteworthy that strongly 

 marked folds radiate from the six angles of the stem, and that six 



" A s 





FIG. xvi. 



Orocyatis 



