THE AMBTJLACEA OF FOSSIL ECHINOIDEA. 447 



of new plates, and the triplets are all reducible to their normal 

 position from which growth-pressure has forced them, on the 

 principle elaborated in the instance of Strongylocentrotiis &c. 



In very young examples of Diplopodia versipora the doubling of 

 the apical pores is seen, and the large compound plates are of the 

 Diadema-type. 



In Diplopodia Mcdhosii, Desor, of Cretaceous age, the replace- 

 ment of the ordinary demi-plate of the ambital plates by a low 

 and curved jDrimary is not uncommon, and both conditions may be 

 seen in the same ambulacrum (fig. 21). 



Fig. 21 (seep. 452). 



Genus Ctphosoma, Agass. 



The genus Cyphosoma is diplopodous ; but the structure of the 

 ambulacra is different from that in Fseudodiadema and Diplopodia. 

 Many of the species are polypores, and four, five, or six plates may 

 enter into the construction of a compound and tubercle-bearing 

 plate. Cyphosoma Konigi. is a very good example, and well- 

 weathered specimens are common. They frequently show the line 

 of partition between the component plates of the compound ones, or 

 rather the lines may be distinctly seen passing from the adoral pores 

 of the obhque peripodia up the boss, and more or less obliquely 

 towards the mamelon. In the majority of specimens two sutui'al 

 lines are seen on the flank of the boss which is opposite the 

 peripodia and near the median line of the ambulacrum (fig. 22). 

 These are perfectly visible in most cases. See figs. 22-25. 



Pig. 22 (see p. 452). 



One of these, that actinally situate, is seen to come from the 



