Pt'olvtheca of the MadreporaruK. 23 



that the epitheca is that part of the skeleton secreted by the 

 edge-zone and left on the sides of the coral as it (the edge- 

 zone) is drawn up with the growth of the coral. This secretion 

 may show periodical wrinkles or thickenings if the withdrawal 

 is intermittent; and it is aIso clearly epithecal, inasmuch as, 

 morphological)}^, it must be regarded as a doubling over of 

 the rim of the prototheca, as can be gathered from the diagram 

 (fig, 14). But this secretion is only one of many, and, 

 moreover, one of the most highly specialized, modifications 

 of the rim of the epithecal cup. Hence while it is quite 

 correct to call it epitheca, it is quite incorrect to define epitheca 

 u\ terms of this single specialization of it. 



It is also clear that if the terra " eutheca " is applied to 

 such cups as those shown in diagrams figs. 1, 2, i, and 5, in 

 which the lip of the prototheca grows straight on, we want 

 some other term to designate a cup in which the bagging over 

 of the soft parts has practically doubled back the edge of the 

 cup, so that the fold adheres to its sides (see fig. 11). But I 

 would suggest that the simple unmodified theca should bo 

 called prototheca, while the term eutheca would be more aptly 

 applied to the theca which has been secondarily attached by 

 a solid pedestal, thickened by the extra matter secreted on its 

 outside, and strengthened and armed by ribs and spines. We 

 might call iliQwaWoi Zaphrentis, Streptelasma,&.c. (diagrams 

 figs. 1-4) " continuously protothecate " and that of Montli- 

 xmliia (diagram fig. 3), or at least of those specimens in 

 which the septa can be seen between the edges of successive 

 saucers, discontinuously protothecate. 



But although this eutheca, with the meaning just suggested, 

 is due morphologically to a doubling of the wall of the proto- 

 theca by the secretion of a layer on the outside of the cup, it 

 can hardly be described as due to a bending over of its rim. 

 I conceive of it rather as due to the rapid bagging over of 

 the soft parts, without at the moment any actual continuous 

 growth of the rim, A true bending over would have been a 

 growth process of the rim itself (sfie fig. lo^). I imagine 

 that only when the soft parts had acquired their new position 

 on the outside of the cup that they commenced secreting the 

 external layer, which is nevertheless strictly a continuance of 

 the rim down the outside and into the basal pedestal. 



This explanation of the morphology and origin of the edge- 

 zone throws an interesting liglit upon a very specialized and 

 morphologically puzzling group, viz. the small highly sculp- 

 tured free Turbinolidse. Their origin can now be understood 

 from diagram fig. 14, if wc suppose that the powers of secreting 



