THE CYST1DEA 



the marginals have extended far over the area formerly occupied by somatic 

 plates, and these latter number from three to six, one of which is much the 

 largest and of irregular shape. Openings other than the median not distinct. 

 Stem of about four alternating rows of plates, often provided with thorn-like 

 processes, and each overlapping its distal neighbour ; total length about 

 equal to that of the theca, the proximal third with a wide lumen. No 

 other appendages observed. Atelecystis (A. Huxleyi], Billings (1858), is 

 imperfectly known ; Anomtdocystis cornutus, Hall (1859), may be con- 

 generic, as usually supposed ; but A. disparilis, Hall, probably belongs to 

 Placocystis (vide iufrci). All the species described by Barrande (1887) as 

 Anomalocijstis are doubtful. The specimens described by Meek (1873) 

 and Wetherby (1879) are separable generically under the latter's name, 

 Enoplnum ; the species are E. lulanoides, Meek sp., and E. Crustacea, 

 Haeckel sp. 1 In all these one traces the gradual diminution in number of 

 plates, especially of somatic plates, and the evolution of the granular 

 ornament into wavy ridges. Some of these also show traces of adoral 

 spines. Belemnocystis is placed in this family by Miller and Gurley (1894), 

 probably with justice, though its exact affinities are obscure. Platycystis, 

 S. A. Miller (1889), is based on a worn Anomalocystid of indeterminable 

 affinities. Placocystis, de Koninck (1869), from the English Wenlock beds 

 of Silurian age (Fig. XIII.), is the most specialised form of this family. It 

 was redescribed by H. Woodward 

 (1880); since then fresh know- 

 ledge has been gained. On either 

 side of the concave face (Fig. 

 XIII. 1) are three marginals 

 (mm) which pass over on to the 

 convex side ; at the columnal end 

 are two marginals, at the oral 

 end are three, and none of these 

 five continue on to the convex 

 side, although corresponding 

 plates occur there. The median 

 adoral marginal of the convex 

 side (Fig. XIII. 2) is the plate 

 M ; its free edge is occasionally 

 denticulate (cf. ridgings in Mitro- 

 tystis). The somatic plates of . 



the concave side are one large (^reconstructed from various specimens in British 

 /o^fral orirl rmn email of ita Museum ; 2, from Brit. Mas., E7545, enlarged ; 3, 



central, and one small at its from the type-specimen, Brit. Mas., KH6&) 

 left upper corner : those of the 



convex side are eleven, viz. two ad-columnal, as in Mitrocystis, supporting 

 one median which does not touch the column as it does in Enoploura ; 

 a transverse row of five, the median of which is small and often quite 

 surrounded by its two neighbours (it is not an anal structure, as supposed) ; 

 a row of three adjoining the adoral marginals. Stem much as in Mitro- 

 cystis. The three marginals that meet at each adoral angle of the theca 

 (Fig. XIII. 3) form an ?~ticular surface (Br) for the support of a spine 



Fio. XIII. 

 Placocystis Forbesianus. 1, concave or supposed 



1 This name must he restricted to Wetherby's Fig. 1, d, e, /, and 1g. 



