120 Prof. P. M. Duncan on the 



ance given under benzule is of a number of low ellipses placed 

 vertically, and they are formed by the adoral sutures of the 

 upper plates and the aboral sutures of the lower plates in 

 succession (fig. 16). 



The Pits. 



The pits at the sutural angles and elsewhere are described 

 in the communication already alluded to (Journ. Linn. Soc, 

 Zool. vol. xvi.). 



The distinctness of the pits in Amhlypneusfes and their 

 slight depth in MesjyiUa, Microcyphus, and Hohpneustes is 

 evident, and A. Agassiz has pointed them out along the 

 lines of sutures as well as at the angles in Pleurechimis and in a 

 species of Amhiypneustes, But the pitting is in excess in 

 TemnoiileuTUs and Salmocis, and is less in the genera just 

 mentioned. The pits near the peristome in Temnopleurus 

 toreumaticus have a large and rather depressed spheroidal 

 sphferidium at their entrance *, and many others have a pedi- 

 cellaria close by. It does not appear that there are any 

 special functions relating to the pits, and the same thing may 

 be said in respect of the deepening, grooving, widening, and 

 depressing of the outer surfaces of the plates close to the 

 sutural lines ; but it is clear that these modifications of the 

 test enable a larger amount of outer test surface to come in 

 contact with water. 



The Articulation and Muscles of the primary Spines. 



The spines are for the most part slightly compressed, and 

 some are very much so ; they have a well-developed milled 

 ring, which is broader than the rest, and which has its milling 

 continuous with the longitudinal fluting of the spine. The 

 spine diminishes in breadth below the milled ring very 

 rapidly, and the hollow cotyloid cavity has a blunt free edge 

 more or less notched. 



The spines are cellular within, with a double series of radi- 

 ating processes and a very narrow circular central space 

 (fig. 28). 



The primary tubercles have a well-formed scrobicule, which 

 slants down from the boss and is often slightly raised exter- 

 nally, so as to present a prominent circular border; it is plain 

 and smooth, except close to the boss, which has a decided 

 crenulation upon it close to the neck of the imperforate and 

 projecting globose mamelon. The cotyloid cavity of the 

 spine fits upon the mamelon, and the notched free edge comes 



* Lov^u, Etudes, pi. x. fig. 38. 



