THE CYSTIDEA 



form, Sphaerocystis (p. 63), or may remain simple, and stretch in the 

 transversal plane. Genera Malocystis, Billings (1858), Chazy Lime- 

 stone, Canada. Theca globular. In the type-species the grooves branch. 

 Amygdalocystis, Billings (1854), Trenton Limestone, Canada. Theca 

 (Fig. XIX.) flattened in plane of food-grooves, and elongate. Grooves 

 never branched. 



FAMILY 6. GLYPTOCYSTIDAE. Rhombifera with stem, theca, and 

 brachioles. The theca composed of five circlets of alternating plates, 

 typically five in each circlet. In first (aboral) circlet, right posterior 

 (r. post.) plate is always fused with right antero-lateral (r. ant.) plate. 

 Anus, with valvular pyramid, between second and third circlets, in right 

 posterior interradius (r. post. IR). Hydropore in adoral circlet, always 

 opposite unpaired arm-groove, and thus defining posterior interradius 

 (post. IR). The trans-sutural foldings of the stereom (pore-rhombs) are 

 restricted in distribution but exaggerated in structure (" pectinirhombs "). 



FIG. XX. 



Actual distribution of pore-rhombs explained by supposed course of gut in primitive 

 Glyptocystidae. The plates numbered as in Fig. XXI. 



One of them invariably unites the left posterior plate of the first aboral 

 row with the left anterior plate of the second row. Mouth central ; from 

 it over the theca radiate food-grooves, primitively 5, by reduction 4 or 2 ; 

 these are bordered by the plates of the adoral circlet, or by plates 

 derived from their proliferation ; and these side-plates bear facets for 

 brachioles of biserial structure. This family is perhaps descended from 

 primitive Echinosphaeridae, in which a natural tendency to the develop- 

 ment of five food-grooves (one ant, single ; two lateral, paired) was 

 accompanied by decrease in number, and increase in size and thickness, 

 of the thecal plates, together with their arrangement in five alternating 

 circlets of five (as in Mimocystis). The diminution of the thecal cavity, 

 we may suppose, pressed the coil of the gut against the body wall ; 

 thus the respiratory function was hindered in the pore-rhombs along this 

 tract, so that they disappeared, while it was thrown more on the remain- 

 ing pore-rhombs, which became highly developed (Fig. XX.). The process 

 continued till only three pair of intensely folded areas were left, one at 

 the base on the side opposite the anus, the others above the anus to 



