THE CYSTIDEA 



seven plates, and a very highly developed stem. Thecal plates have 

 strong radiating ridges, marginal concentric ridges, and suture margins 

 toothed on the inside in correspondence with the ridges. Since this struc- 

 ture is not unlike that in Comarocystis, the genus is provisionally placed here. 

 FAMILY 3. MACROCYSTELLIDAE. Rhombifera in which the theca 

 consists of three or four circlets of plates, subjected to a more or less 

 regular pentamerism. The stereom is strongly radiately ridged or folded, 

 but no so-called "pores" or pectinirhombs are developed. Brachioles 

 are borne by the upper circlet of plates, and within these there may have 

 been tegminal plates over the mouth. There is perhaps no intimate 

 connection between the two genera referred to this family. But they 

 are both Cambrian, and show an early development of that tendency to 

 reduce the number of plates, which eventually evolved the Glypto- 

 cystidae from a different branch of the Rhombiferi. The Macrocystellidae 

 were probably derived from Eocystidae without passing through an 

 Echinosphaerid stage. Genera Macrocystella, Calla- 

 way (1877), Upper Cambrian, Shropshire. Theca 

 seen from the side (Fig. XVIII. 1) shows four circlets 

 of plates, apparently five in each circlet. Those of 

 the aboral circlet are low and pentagonal ; those of 

 the second and third circlets hexagonal and rela- 

 tively large ; those of the fourth circlet about half 

 the size of those in the third, sub-pentagonal, and 

 each bears a brachiole. These almost immediately 

 bifurcate, making ten branches in all, about as long 

 as the theca is high, and apparently biserial : covering- 

 plates are distinct. There were probably tegminal 

 plates above the origins of the brachioles. Thecal 

 plates strongly marked with radiating folds, which 

 divide the surface into triangles ; between them are 

 smaller folds. No fine rhomb structure is seen. 

 Anus unknown. Stem rapidly tapering, about half 

 as long again as total length of crown ; proximal 

 columnals low and imbricating, with very .wide 

 lumen ; distal columnals long and narrow. Mimo- 

 cystis, Barr. (1887), Ordovician, Bohemia, does not, so 

 far as can be gathered from the published description, 

 differ from Macrocystella in any essential. Lichenoides, 

 Barr. (1846, 1887; Pompeckj, 1896; syn. Licheno- 

 cystis, Haeckel), Cambrian, Bohemia and Bavaria. 

 Theca composed of rounded plates of very different 



. , , 



At the base 



are five to twelve minute P lates > indicating absence of 

 surfaces ; 5, single plate stem and probably of fixation, at all events in adult. 



of a large specimen (Brit. A -L J.T i i. r .c 11 i 



Mus., E752S), nat. size. Above those is a circlet oi five irregularly pentagonal 

 large plates. Resting on, and to a certain extent 

 alternating with these, are six or seven plates of similar size. A circlet 

 of smaller plates, alternating with the last mentioned, forms the summit, 

 and bears about eight biserial unbranched brachioles. No anus observed. 



FIG. XVIII. 



Macrocystella Afariae. 

 1, from side (recon- 

 structed from Brit. 

 Mus., E7523 and E7524), 



2-4, portion of a sizes, but semi-regular in arrangement. 



