450 



PELMAT0Z0A. 



closed by an exceedingly thin calcareous membrane, while they in- 

 variably traverse the entire thickness of the plates. Their arrange- 

 ment differs much in different types of the Cystideans, and they 

 afford therefore important evidence in classification. In certain 

 forms (such as Glyptosphczrites, Gomfihocystites, &c.) the pores are 

 conjugate, or are united in pairs, the individual plates possessing 



Fig. 321. — A, Part of the calyx of Eocystites ? longidactylus, from the Cambrian rocks of North 

 America, enlarged, showing pores passing between adjoining plates ; b, Plate of Glyptosphcerites 

 Lenchtenbergi, enlarged, showing double pores ; c, Plate of Echinosphcerites aurantium, show- 

 ing diffused pore-rhombs, enlarged, from the Ordovician rocks of Russia, (a is after Walcott.) 



several of such pairs (fig. 321, b). In other forms, the pores are 

 arranged in rows, a row on one plate being united by grooves or 

 fissures, sometimes on the exterior of the calyx or at other times 

 on its interior, with a corresponding row on an adjoining plate. 

 These rows of connected pores (fig. 322) are arranged so as to form 

 lozenge-shaped or rhombic figures ; in such a manner that one-half of 



Fig. 322. — a, Pore-rhomb of Glyptocystitcs multiporus (Billings) ; b, Pore-rhomb of Pleuro- 

 cystites (Billings) ; c, Two plates of Callocystites Jewettii (Hall), showing the pore-rhombs (p). 

 All enlarged. 



each rhomb belongs to one plate and the other half to its contiguous 

 neighbour, while the grooves connecting the two component rows of 

 pores pass across the line of suture between the two plates, either 

 externally or internally. In some types, as in Echinosphcerites (fig. 

 321, c) and Caryocystites, these "pore-rhombs" are very numerous, 

 and are present upon nearly all the plates of the calyx. In other 



