THE FAMILY ABBACIADiE. 49 



bourhood of the apex, are compound plates as fa the Triplecliinidae, 

 that is to say, each is built up of several poriferous plates. The 

 groiiping of these plates and their relations inter se are, however, 

 very different in the forms under notice and the group to which 

 we have just referred. 



In most of the species o^ Arhacia (e. g. A. stellata, A. punctulata, 

 A. pustulosd) there are three poriferous plates in each compound 

 ambulacral plate. The adoral and aboral plates of this triplet 

 are small demi-plates, and occupy scarcely more than half the 

 length of the ad- and aboral margins of the compound ambu- 

 lacra! plate respectively ; and the mean depth of these plates may 

 be roughly stated as being about one third of the depth of the 

 whole compound ambulacral plate. It thus follows that the 

 greater part of the compound ambulacral plate (approximately 

 two thirds) is formed of one large primary poriferous plate» 

 which occupies the whole of the inner half of the compound 

 plate, and which also extends as a narrow strip between the two 

 small demi-plates above mentioned, its outline being more or less 

 spade-shaped, or fancifully suggestive of an old-fashioned sugar- 

 spoon. 



The forms and posture of these plates differ more or less in the 

 different species of Arbacia. After the preliminary remark that 

 the compound plates may be best studied at a little distance 

 above the ambitus, and from thence to the peristome, we proceed 

 to examine Arlacia pustulosa. 



In this species the whole ambulacral plate is subject to a more or 

 less considerable adoral flexure at the commencement of the pori- 

 ferous region, which will be seen on referring to fig. 1 (PI. II.). 

 The aboral poriferous plate, which is a demi-plate, has a rounded 

 extremity internally ; the suture commences almost atarightangle 

 to the aboral margin of the whole ambulacral plate, mounts the 

 boss of the primary tubercle, curving meanwhile, and the bend 

 being completed near the summit of the boss, the suture thence 

 proceeds in a direct but inclined line to the outer end of the 

 compound plate. The aboral demi-plate is somewhat enlarged 

 at its outer end, as shown in the figure. The adoral demi- 

 plate, on the other hand, is enlarged at its internal end, and in 

 consequence presents somewhat of a pyriform outline ; its suture 

 mounts the base of the boss of the primary tubercle, is rather 

 rapidly bent with a w^ ell -rounded curve until at a right angle to 

 its initial direction, and thence proceeds in a slightly sigmoid 



LINN. JOUEN. — ZOOLOGY, VOL. XIX. 4 



