part 1] THROirmi the andes of peru and boliyia. 43 



for example, of the specimen shown in PI. I, (ig. 8, with Zaphrentis 

 disjuncta as figured by Mr. Carruthers, 1 shows that the two 

 possess the following features in common : — Longitudinal ribbing of 

 the epitheca ; cardinal fossula on the concave side of the corallum ; 

 22 major septa ; septa Amplexoid, thickened at their inner ends, 

 and convex to the cardinal fossula ; no minor septa ; simple tabulse. 



The development of Amplexoid septa in a late stage of the 

 phylogenetic history of the gens must necessarily have seriously 

 lessened the structural strength of the corallum. and the lengthening 

 of one of the septa, in this case the counter-septum (which, from 

 its position, is naturally most fitted for the purpose), to form a 

 central plate thickened at its end into a rod-like columella, would 

 materially aid in counteracting any such weakness. It is possible, 

 then, that the extinction of the delanouei gens, foreshadowed by 

 this catagenesis or simplification of structure, may have been 

 delayed by the acquisition of a Lophophylloid columella. 



The causes of variation are too little known to admit of an 

 adequate proof of such a theory; but the phenomenon of parallel 

 development in many Avonian corals suggests that these forms of 

 life were readily adaptable to external conditions. 



Perspecialization in any gens is characteristic of the stage 

 immediately prior to extinction, when its capacity for survival is 

 limited by its power to change ; and this doubtless is largely 

 influenced by environment, migration to new surroundings tending 

 to give a new lease of life. 



It might reasonably be argued, however, that the extension of 

 the counter-septum more than half-way across the corallum may be 

 taken to imply the retention of a primitive character rather than 

 the acquisition of a new one, for in the protoseptal stage of the 

 Rugosa the cardinal and counter-septa are continuous one with the 

 other, and the point of separation need not necessarily be central, 

 a feature often seen in such forms as Caninia cornucopiee and 

 Zaphrenfis omaliusi. It is the secondary thickening of the 

 counter-septum, however, rather than its length, that is the chief 

 factor in forming the columella, and a study of the ontogenetic 

 development of the coral, made by grinding down a number of 

 individual specimens, shows that its special modification for this 

 purpose does not take place until a comparatively late stage of 

 growth, when the septal plan is already well established. The 

 central rod-like columella, moreover, often almost circular in cross- 

 section, appears in some cases to be added to the extreme tip of the 

 counter-septum, while in others it is formed by lateral thickening. 



In discussing the question of convergence or coeval assimilation, 

 it is not without significance that several of the corals here de- 

 scribed, which possess in common this feature of an extended and 

 swollen counter-septum, differ one from the other in other points of 

 structure, and appear to have been derived, if not from different 

 genera, at least from different species-groups. If, then, I have 



1 Q. J. G. S. vol. lxvi (1910) pi. xxxvii, fig. 7«. 



