Anatomy of the Teranopleuridff". 117 



plate ; the middle plate {h) is a low narrow demi-plate, and 

 the adoral pair of pores is in a large primary plate (a). 



Holopneustes. 



The nature of the ambulacral plates of Holopneustes purpu- 

 rescens, Liitk. sp., is much easier to comprehend than that of 

 the other and broad poriferous zoned species, H. porosissimus^ 

 Agass. But the same method of examination must be em- 

 ployed as in other polypores, and when it has been mastered 

 in the first-named species tlie difficulty vanishes with regard 

 to the apparent confusion of the plates in the other form. 

 The rule must be followed which enables the adoral pair of 

 pores or their peripodium to be distinguished ; and it must be 

 remembered that in the great majority of instancss the pair is 

 nearer the ambulacral median line than the other pairs of a 

 compound plate. 



In Holopneustes purpuvesceas (figs. 14, 15, 16) the ambu- 

 lacral plates are low and broad, and usually there are double 

 plates near the ambitus (fig. 14) consisting of two vertical 

 sets of triplets combined in a geometrical plate ; elsewhere 

 the plates are single, or there may be an alternation of single 

 and double plates. The test is rather thin and the poriferous 

 zone is rather broad, the peripodia being triserial in arrange- 

 ment. The pairs are close vertically and rather distant hori- 

 zontally. One vertical row of pairs is very regular and is 

 internal, that is nearest the interporiferous area, and each 

 pair is in the adoral plate (a) of a compound plate. This adoral 

 plate is a primary, and forms most of the interporiferous part 

 of the geometrical plate ; it extends to the median suture ; but 

 it may be excluded from that part of the poriferous zone 

 which is close to the ambulacro-interradial suture (figs. 14— 

 16), or, as is the case near the apical system, it forms all the 

 adoral part of the compound plate and reaches the ambulacro- 

 interradial suture (fig. 15, a). 



The outer vertical series of pairs of pores is also a very 

 regular one and consists entirely of those belonging to the 

 middle components (5, figs. 14 and 16) of compound plates, the 

 component plate being a narrow demi-plate. Near the apical 

 system the little demi-plate (^, fig. 15) is separated from the 

 adoral suture of its geometrical compound by a low part of 

 the primary just noticed j but usually there is no such inter- 

 val, and the expansion of the demi-plate has caused the 

 exclusion of the portion of the primary immediately adoral to 

 it (figs. 14-16). It is this blotting-out of part of a plate 



Ann. &. Mag. N. Hist. Ser. 6. Vol. i. 9 



