430 Prof. S. Lov^n on the Structure of the Echinoidea. 



approximated angles of equal size. These two pentagons are 

 so placed that the side l.aN.h stands towards the unpaired 

 interradium, and towards the madreporite's interradium the 

 side II. h III. a, which is homologous with this, — that the four 

 sides which in the two pentagons enclose angles of equal size 

 but not homologous, are parallel two and two, but the sides 

 which are common for the two unequal but homologous angles 

 intersect each other, — and, moreover, so that the two pentagons 

 together form with their outer lines a figure which is symmetrical, 

 not in relation to that diameter of the stoma which coincides 

 with the antero-posterior axis of the animal, but only in relation 

 to a diameter, a o), which passes through the point where 

 the last-mentioned sides intersect each other, so that all lines 

 which unite the homologous angles of the two pentagons 

 (namely, IV. a with IV. h, III. h with V. «, V. h with III. o, 

 I. a with II. J, and I. h with II. a) are reciprocally parallel and 

 perpendicular to the same diameter — and that consequently, if 



Fig. 3. 



Buccal area in a young Brissopsis lyrifera : sph. sphferidia, 



one pentagon is turned round this diameter as upon an axis, it 

 coincides with the other. In the same way we get two similar 

 pentagons in the pentagonal peristome with rounded angles of 



