The Annals and magazine of natural history; zoology, botany, and geology . gons areso placed that the side stands towards the unpairedinterradium, and towards the madreporites interradium theside II. h III. a, which is homologous with this,—that the foursides which in the two pentagons enclose angles of equal sizebut not homologous, are parallel two and two, but the sideswhich are common for the two unequal but homologous anglesintersect each other,—and, moreover, so that the two pentagonstogether form with their outer lines a figure which is symmetrical,not in relation to that diameter
The Annals and magazine of natural history; zoology, botany, and geology . gons areso placed that the side stands towards the unpairedinterradium, and towards the madreporites interradium theside II. h III. a, which is homologous with this,—that the foursides which in the two pentagons enclose angles of equal sizebut not homologous, are parallel two and two, but the sideswhich are common for the two unequal but homologous anglesintersect each other,—and, moreover, so that the two pentagonstogether form with their outer lines a figure which is symmetrical,not in relation to that diameter of the stoma which coincideswith the antero-posterior axis of the animal, but only in relationto a diameter, a o), which passes through the point wherethe last-mentioned sides intersect each other, so that all lineswhich 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 andperpendicular to the same diameter—and that consequently, if Fig. Buccal area in a young Brissopsis lyrifera : sph. sphferidia, one pentagon is turned round this diameter as upon an axis, itcoincides with the other. In the same way we get two similarpentagons in the pentagonal peristome with rounded angles of Prof. S. Lovdn on the Structure of the Echinoidea. 431 the young- Spatangus (see fig. 3); and it is clear that this is thecase also in the typically equally pentagonal peristome of theClypeastridai and Cassidulida3. The diameter a co passes, ifproduced, through ambulacrum IV. and interradium 1. The interradia are rows of plates of the perisome, differentfrom the ambulacra in growth and movement. If, by boilingin solution of soda, we carefully separate their plates in Sj)a-tangus ^urpureuSj Brissopsis lyrifera, and Echinocardiumovatum^ we find that many of them are furnished at their adoralangles with a semilunar lamella,which projects within the aboralmargin of the prece
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