112 Prof. P. M. Duncan on the 



There appears to be an amount of superficial growth of test 

 which overrides the plates here and there in the ambulacra of 

 MicrocyphuSj and this has to do with the abolition of the 

 suture, as well as a remarkable blotting-out of plates, which 

 will be mentioned further on. 



The Sutures as seen in Decalcified Specimens of 

 Temnopleurus toreumaticus. 



After decalcifying in dilute hydrochloric acid in spirits of 

 wine, washing, staining with hasmatoxylon, placing in 

 absolute alcohol^ clearing and mounting, the sutures between 

 the interradial plates become beautifully distinct, on account 

 of the presence of a lamina or ribbon-shaped process of 

 reticular connective tissue, which dips down between the 

 contiguous edges of the plates. It appears to be that part 

 of the connective tissue of a plate or plates in which the 

 knobs and sockets, made up of very reticular spicules of car- 

 bonate of lime, are deposited as the test grows. In some 

 parts, especially in the apical region, the suture between two 

 plates is recognized by a cross- layering close to the divisional 

 line. 



The Structure of the Amhulacral Plates. 



The ambulacral plates of all the Tenmopleurinas are com- 

 pound, and the pairs of pores are in triplets, which vary in 

 their obliquity, vertical closeness, and horizontal distance. 

 The least complication is seen in the genera Temnopleurus 

 and SalmaciSj and the greatest is observed in Microcyphus 

 and Holopneustes. It is evident that two factors produce 

 complexity, and they are diminution of the vertical dimen- 

 sions of the compound plates and thickening of the test during 

 growth. 



Simple as the ambulacral plates of Temnopleurus and Sal- 

 macis are, they afford a good introduction to the study of the 

 more complicated forms. 



Temnopleurus and Salmacis. (PL XI. figs. 2, 3, 4.) 



In all the species of these genera the ambulacral plates are 

 in compound geometrical forms, which are low and broad in 

 Salmacis and taller and narrower in Temnopleurus. In all 

 species the compound plates are made up of three components ; 

 the upper or aboral component (c) is a long low primary, and 

 the middle one {b) is a low short demi-plate with a very curved 



