THE CYSTIDEA 



bilateral symmetry, and therefore started from the usual sack-like form. 



Genera Trochocystis, Barrande (1859-87 ; syn. Trigonocystis, Haeckel), 

 Cambrian, Bohemia, France, Spain (Fig. XI.), is 

 the most primitive ; its theca is bounded by 

 twelve stout marginals (mm), conspicuous on one 

 side more than on the other, and forming a 

 circular, elliptical, or subtriangular frame ; the 

 enclosed space on either side is occupied by a 

 mosaic of 80-160 somatic plates, hexagonal 

 except where truncated by the marginals. At 

 the oral end of the frame three openings, nearly 

 in the broad median plane, pass from the thecal 

 cavity through or between the marginals to the 

 exterior; the middle opening (? hydropore and 

 gonopore) is widest, and is protected by a 

 hood-like projecting plate (M), (? madreporite) ; 

 of the other two openings, one (0) in the broad 

 plane is the wider (? mouth), the other (As) 

 slightly out of that plane is smaller (? anus) ; 

 they appear to be connected by a canal (? for 

 reception of gut) running round the thecal 

 cavity on the inside of the marginals. At the 

 aboral end of the frame the marginals pass 

 into a short, tapering stem (St) of subtri- 



is removed showing the interior, angular section, composed of rows (3 or 5?) 



The arrows show the supposed . ,. . . , . n . 



direction of the gut. The letters of alternating ossicles, its lumen commum- 



are^ explained in the adjoining cating with tlle thecal cav j ty . Trochocystis 



may be regarded as a differentiation from 



such a form as Aristocystis by lateral compression, so that its broad 

 median plane is morphologically the 

 sagittal, and the flat sides are the 

 primitive right and left, the pro- 

 jecting plate M being on the left side. 

 Mitrocystis, Barrande (1887), Cam- 

 brian and Ordovician (Fig. XII.), 

 has twelve marginals (mm), but on 

 the (left ?) side that corresponds with 

 the convex side of later forms, the 

 junction of the stem with the somatic 

 plates lies between two of them ; 

 while the median adoral plate of 

 this side (M) is vertically grooved on 

 the interior, but exteriorly resembles 

 the somatic plates, which on this 

 side, though larger and fewer (50-60) 

 than in Trochocystis, still form a 



mosaic of hexagons; two, adjoining of upper part of the latter, showing the folded 

 the stem, are larger than the others. plate - V " (After Barra " de -> 

 On the other side, which corresponds with the concave side of later forms, 



FIG. XI. 



Trochocystis bohemicus, re- 

 stored on the evidence of Bar- 

 rande's figures. A part of the 

 theca on the right of the figure 



FIG. XII. 



Mitrocystis mitra. I, from supposed right 

 side ; 2, from supposed left side ; 3, interior 



