THE FAMILY ARBACTAD^. 41 



The three pairs of pores to these tubercles are placed in an 

 arc, and each pair intrudes upon the base of the tubercle, and the 

 adoral pore may almost be hidden by it. The pores are small, 

 close, separated by a nodule, and they are surrounded by a peri- 

 podium except near the apex. They are placed obliquely, and 

 their direction is outwards and decidedly upwards, so that the 

 aboral pores are really worthy of the name, and this obliquity 

 increases in the actinal great tubercle. The pairs of pores are 

 wide apart, and the distance of the first and second pair is slightly 

 greater than between the second and third. The first pair is 

 placed on the aboral and outer shoulder of the tubercle, and a 

 line drawn across the tubercle from the pair inwards would pass 

 aborally to the groove around the mamelon. The middle pair is 

 either on a level with the middle of the mamelon, or slightly 

 adorally, and the lower pair is close to the lower and outer 

 edge of the base. 



Each great tubercle and the plate which it covers is really a 

 triple combination ; and the sutures of the three plates are some- 

 times, but not invariably, slightly indicated as faint groove-like 

 marks passing up the side of the boss from the adoral pores of 

 the pairs. There is no trace of a passage of sutural lines over 

 the top of the mamelon or its base, and it is evident that the 

 ordinary method of union of triple plates seen in the Triplechinidse 

 does not occur (PI. I. fig. 9). 



On cutting through a specimen of Goehjpleurus Maillardi 

 preserved in alcohol, and removing the delicate investing mem- 

 brane from the inside of the test, the markings of the su- 

 tures of the triple combinations become very distinct to view 

 (PI. I. fig. 8). After drying, the more delicate sutural lines 

 disappear ; but there are some media which restore the ap- 

 pearance for a while. It becomes evident on a most super- 

 ficial examination that the composition of each of the great 

 tubercle-bearing plates at the ambitus is almost the same as that 

 of the fossil species. The sole difference is in the direction of 

 the inner sutural lines of the aboral and adoral demi-plates of 

 the compound plates. And indeed the difference is not a perfect 

 one, for the almost vertical direction of these lines is noticed in 

 some parts of the fossil forms. The difference is of no import- 

 ance. As in the fossil species, each compound plate covered 

 by a great tubercle consists of three plates, of which the aboral 

 and adoral are small, do not reach the median line, and are 



