316 



ZOANTHARIA. 



Fig. 199. — Calices of Alveolites sub 

 orbicularis, Lam., greatly enlarged, 

 showing the single septal ridge. Devon 

 ian, Eifel. (After Goldfuss.) 



or three elongated tooth-like ridges (fig. 199); but in some species 

 the corallites possess strong ascending calcareous spines, the nature 

 of which is different. The mural pores are uniserial and of large 

 size, and the tabulae are complete and more or less horizontal. 



The species of Alveolites usually have 

 a branched or massive corallum, the 

 latter usually formed of superimposed 

 crusts ; and they are found in the 

 Silurian and Devonian deposits. An 

 allied genus, with a similar geological 

 range, is Ccem'tes, in which the coral- 

 lites are greatly compressed, and are 

 much thickened towards their mouths. 

 Another group is constituted by 

 the genera Pleurodictyum and Michel- 

 inia, in both of which the tabulae 

 are more or less "cystoid" or ves- 

 icular, and the mural pores are 

 very irregular. It is, indeed, very 

 doubtful if these two genera can 

 be separated, and in case of their 

 union the name of Michelinia will have to be abandoned. In 

 Pleurodictyum (fig. 200) the corallum is discoid ; with a flat or con- 

 cave base, covered with an epithecal plate, and attached at one point 

 to a foreign body. The corallites are polygonal, and diverge from 

 the centre of the base, those on the circumference being nearly hori- 

 zontal or even bent downwards, while the median ones are more or 

 less perpendicular. The mural pores are numerous and quite irreg- 

 ular ; and in one species there is the singular feature that the mural 

 pores of the under sides of the peripheral corallites pass directly 

 through the basal plate of the colony, which thus becomes perforated. 

 The tabulae often inosculate, and become to some extent vesicular, 

 but these structures are comparatively few in number, and the " cys- 

 toid" character of the tabulae is thus little marked. The genus 

 Pleurodictyum is wholly Silurian and Devonian, so far as known, 

 if Michelinia be excluded from it. The best known species is the 

 P. problematicum of the Devonian rocks of Europe, the singular 

 discoid casts of which (fig. 200, a) are commonly found in the 

 Lower Devonian of the Eifel. In these casts the conical columns 

 (fig. 200, b) represent the moulds of the visceral chambers of the 

 corallites, and the little cylindrical rods connecting these are the 

 result of the infilling of the mural pores. A singular feature in these 

 casts is, that there is generally to be seen a curious cylindrical 

 twisted body (fig. 200, a) in the centre of the base. This "worm- 

 like body" has been very generally recognised in specimens of 



