20 SVEN LOVEN, ECHINOLOGICA. 



The iiew plate makes its first appearance in the angle 

 betweeii the costal and the radial, as a minnte section of a 

 circle, with the curvature eontiguous to the preceding plate. 

 It is developed by little and little of its superficial extent, 

 and, so to say, alla prima ■. The adoral margin appears first. 

 and very soon it exhibits a beginning semicircle of verruculse, 

 each of which bears the rudiment of its blade-like spinule. 

 These spinules grow very rapidly and soon lie down converging 

 to protect the central part now coming forth, the cone, which 

 in this stage rises directly from the circumference without 

 any scrobicular depression. In this state the yonng plate is 

 nearly quadrangular. It now seems to recede from the calyx, 

 ac(juires its fifth inner angle, the sixth, the external, being 

 adapted to the moving, nndulated ambnlacrnm, obtiise and 

 rounded. Meanwhile the mamelon and spine are forming. 

 Before the last third part of the new plate has appeared they 

 are easily seen developing, as described by PiiouHO,^ if the co- 

 vering spinulse. are drawn aside. Each of these begins as a 

 low, circular elevation. A series of seetions shows that the 

 mamelon, rising from the cone, and flat at tlie top, is covered 

 with a low and thin cap, the first rudiment of the spine. 

 When nearly the whole of the plate has been formed and the 

 mamelon is almost central and fully encircled by the multi- 

 plied spinulai, the cap has grown convex and its underside 

 concave, fitting to the mamelon, and in either there is a cen- 

 tral pit for the inter-articular ligament. Then the cap thick- 

 ens and becomes higher, while the flanks of the cone become 

 slightly concave, au impression all arouud renders it exter- 

 nally distinct from the mamelon which takes a semiglobular 

 form, and the rudimental spine, now twice as high and tape- 

 ring upwards, bulges out below to form the head and ring, 

 and the shaft soon shoots up and increases forcibly. In a 

 specimen 50 mm. in diameter the weights, in grams, of the 

 five uppermost plates and their spines are respectively and 

 relatively : 



a 8 Plate, Gram 0,0425: Spine, Gram 0,oooi -^ 1 ;0,oo24 



b 7 '' ■» 0,1282: • i> 0,004!» = 1 : 0,0382 



a 7 ■■> » 0,2095: » » 0,629] = 1 : 3,002!) 



b 6 •' > 0,2350: » » 0,9423 = 1:4,01 



a 6 ' 0,2510 : ' » 1,2640^1:5,0858. 



Recherches s. la Dorocidaris papillata, p. 4(3. 



