KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 19. N:0 7. 35 



III b, IV a, v 6, ahvays difier in a mai'kecl inaniier from tlie othei- five: I b, II b, 

 III a, IV 6, V a, be it in size, in outline, or in the number of their pedicellar pores, 

 and the two sets follow each other, all avound the stoma, everywhere in the same 

 order. And this invariably recurring biformity of the ambulacra in their peristomal 

 beginnings, of a never failing validity throngh the Avhole of the Echinoidea, recent and 

 extinct, that have been examined on this point, is also more or less distinctly recognis- 

 able all through the entire length of each ambulacrum, and more so in the older 

 types, in which the seemingly radiate disposition of the parts is predominant, than 

 in those of a låter date. In Cidaris the plates of the rows I b, II 6, III a, IV b, V a, 

 in every ambulacrum are in advance of those of the rows I a, II a, III h, IV a, V b^ 

 by an entire plate or half a one^), and in the very young and minute Toxopneustes 

 drcebachensis, the seventh plate is begun in each of the rows I 6 ... V a, while there 

 are yet only six in each of the rows I a . . .Y b'). But in the types of låter appearance, 

 the Neonomous, m which the radiate form largely makes way for a decidedly 

 bilateral disposition, as in the Cassidiilidas, CollyritidEe, Holastridae, Spatangidse, the ten- 

 dency towards a gradually more and more decided syminetr}' in external outline be- 

 tween the two lateral ambulacra of the trivium, II and IV, and between the two bi- 

 vious, I and V, seems to invalidate the biformity of their composition, and leave it in 

 full force solely in the peristome. There it, therefore, continues from the antique 

 Cidaridaj till it all but vanishes in the Clypeastrida^, of Tertiary origin, always suggesting, 

 in a manner not yet understood, the previous existence of a skeletal axis diverging 

 from the actual antero-posterior one, being at an angle with it, and dividing length- 

 wise the ambulacrum IV and the opposite interradium. If iive homologous points in 

 I a, II a, III b, IV a, V b, are connected by straight lines, and also homologous 

 points in I b, II b, III a, IV b, V a, two irregular pentagons are inscribed within the 

 peristome, disposed in sueh a manner that each of them is divided by this axis, ccco, 

 into two parts, one trapezoid and the other pentagonular, the areas of which are equal; 

 and that, if one of them is made to revolve half a circle on the axis «a), it covers 

 the space left by the other'). 



It results from the ai'rangement thus detailed, that the two bivious ambulacra are 

 symmetrical towards one another with regard to the disposition of their constituent 

 elements. Within the trivium the front ambulacrum, III, in so far has a medial cha- 

 racter, as by its peristomals it is symmetrical on one side towards the ambulacrum II, 

 on the other towards the ambulacrum IV. But these latter, the two lateral ambulacra 

 of the trivium, are, Avith regard to the disposition of their elements, unsymmetrical 

 towards each other, thus testifying to the inherent obliquity of the Echinoidean ambu- 

 lacral system. In both the principal types of the Echinoidea, however, this obliquity is 

 outwardly concealed: in the Archajonoinous, the dentiferous forms, Cidaridae and Echi- 

 nidse, by the apparently quite circular and radiate arrangement of the skeletal systems, 

 the central position of the mouth, and the verticality of the stomato-proctic axis, all 

 correlatively bound up with one another and rigidly maintained; in the låter, the 



') Ktudes, p. 30. 2) ib.^ p. 21, pl. XVII, Hg. 140. •*) Ib., p. 37, pl. XVII, fi^, 140. 



