THE CYSTIDEA 47 



mouth (0) is surrounded by five large plates, forming a slight projection, 

 somewhat eccentric. The anal pyramid (As) has five plates. The hydro- 

 pore (M), a little to the right of the line joining mouth and anus, was 

 covered by a small pyramid of three plates, the impressions of which 

 on the internal cast (the only part preserved) have been regarded as three 

 openings. The canals in the thecal plates are more numerous near their 

 margins. Certain species, in which the plates seem more rounded, not 

 closely apposed, and perhaps without canals, in which the anal pyramid 

 had four plates, and in which the hydropore was not tripartite (i.e. had no 

 valvular plates), have been separated by Haeckel (1896) as a genus, Amphora- 

 cystis. Piracy stis, Barrande (1887), Ordovician, 

 Bohemia (Fig. VIII.), had a pear-shaped theca trun- 

 cate below for fixation (St\ but still without true 

 stem. The anal pyramid had six plates (As) ; other- 

 wise it was much like Deutocystis. The regularity 

 of the adoral plates in these two genera suggests 

 that they may eventually prove to be early forms of 

 Diploporita or Rhombifera. 



The Lower Niagara rocks (Silurian) of Indiana 

 and neighbouring states have yielded numerous forms 

 resembling Aristocystis in external appearance and 

 structure of theca, but with an ambulacral system 

 apparently presenting three grades of organisation. 



They have all been described under the generic Plrocystis pirum, re- 



Tr , , ., fj- , , . . , stored outline on the 



name Holocystttes or Holocystis, a name previously evidence of Barrande, 



given to a coral, and therefore bound to vield to the P.V 2 ?v Lettering as in 



*"' Fi (p . II. 



alternative Megacystis, Hall (1864-65). Some of the 

 so-called species described by Hall and S. A. Miller seem to agree with 

 Aristocystis in the entire absence of arms and food-grooves, in the similar 

 position and structure of mouth ("ambulacral orifice," S. A. M.), and anus 

 ("mouth," S. A. M.), while a hydropore ("anus," S. A. M.) is often ob- 

 servable, and occasionally a fourth opening (? gonopore) ; the positions 

 of the two latter are at varying distances between mouth and anus. 

 Miller has described other species with similar structure^but with four 

 or five of the plates surrounding the mouth raised into elliptical facets, 

 apparently for the support of spines like those of Placocystis (p. 51); 

 no groove connects these facets with the mouth, although in some species 

 the mouth assumes a tetragonal or pentagonal outline, with angles 

 directed towards these facets. The third and higher stage of organisa- 

 tion, possibly developed from this one, is seen in Holocystites gyriiin.*, 

 Miller and Gurley (1894), and must be referred to the Sphaeronidae 

 (see p. 72). 



FAMILY 2. DENDROCYSTIDAE. Amphoridea with a single oral skeletal 

 process, theca composed of numerous irregular plates, extending below 

 gradually into a stem. The single genus, Dendrocystis, Barrande (1887), 

 Ordovician, Bohemia and Russia (Fig. IX.), has a theca in shape and 

 intimate structure not far removed from that of Aristocystidae ; of equal 

 thinness all over, its plates irregularly polygonal, and their strands of 

 rnesostroma not so well-defined. The following differences are of great 



