440 PROP. p. M, DITN-CAN 02f THE STETJCTXTEE OF 



Fig. 14 (see p. 452). 



The tubercle covers nearly the whole of the three plates which 

 compose the compound plate, and even the peripodia are on its 

 slope. 



The tubercle, as is well known, is large and tall, has a sloping 

 boss, a wide crenulated ridge and groove, and a large perforated 

 mamelon. There are three pairs of pores surrounded bj' as many 

 peripodia in immediate relation with the plate, and they are rather 

 distant and in an arc. (A pair situated adorally to the others is in 

 connexion with the compound plate placed immediately actinally. 

 Again, a pair which is on a line with the aboral edge of the tubercle 

 belongs to the plate above.) On examining most specimens the only 

 trace of a suture between any of the component plates is seen very 

 generally as a depressed line on the side of the boss towards the 

 median line of the ambulacrum and passing towards the aboral and 

 inner angle of the compound plate or rather of the tubercle. The 

 direction of the hue is apical and to the median line, and it 

 reaches this last either slightly or considerably below the aboral 

 angle of the compound plate at the vertical suture. But in many 

 weathered specimens there is another and distinct suture visible, 

 and it passes actinally from the crenulated edge over the adoral face 

 of the boss, and it may reach the transverse suture with a gentle 

 curve. On the poriferous side of the tubercle the first-mentioned 

 suture is seen to be in relation with the highest of the three 

 peripodia of the plate, to commence in the line of groove passing 

 adorally to the obliquely placed first pair of pores, no. 40, and to 

 reach up the side of the boss to the crenulated ridge, and then to 

 cross the boss towards the median line. The direction of this suture 

 may be in a right line or in a slight curve with the convexity 

 looking actinally. The suture joins the aboral and central plates of 

 the triplet, 'and it leaves the mamelon adorally and pursues a more 

 or less oblique course. It is clearly touched by the adoral pore of 

 the pair. 



The shape of the suture and its direction determine to a great 

 extent the shape of the aboral plate of the combination, and this 

 is a primary plate with the poriferous area higher in vertical 

 measurement than the opposite extremity, and with the inter- 

 mediate part the highest of all. 



The next pair of pores, situated adorally to the last, are also 

 obliquely placed, and on the edge of the boss (no. 41), and the 

 adoral pore is in contact with a suture which passes inwards and up 



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