KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7. 81 



radials I and V, again presents itself. It is in Ponrtalcsia Jeflfreysi, P. laguncula, P. 

 ceratopyga that the disordered and unsettled condition of the system is brought to a' 

 degree not reached in any other Echinoid, recent or fossil. The srnallness and irregu- 

 lar outline of the whole, the frequently total obliteration of the sutures, the abortion 

 of the radials, the displacement of the costals 1 and 4, consequent upon the general 

 forward movement of the parts, and accompanied, no doubt, internally by a displace- 

 ment of the sexual organs, the sevei'ance of one of the costals, and the abnormal con- 

 tiguity of 1 and 4 to the terminations of the paired trivious ambulacra, otherwise the 

 legitimate site of the radials, all these more or less abnormal features are as many 

 signs of approaching ruin. During the long range of geological time that lies between 

 the Triassic Tiarechinus with its ancestral, large, and regularly radiated calycinal sy- 

 stem, and Pourtalesia with that same system degraded, shrunk, and dismembered, the 

 evolutional process is marked by the successive appearance of forms unseen before, 

 each bearing in the condition of the calyx the criterion of its geological age. The 

 large, regular and intact calyx of the Endocyclic forms, with five costals and five radi- 

 als equally balanced, points back to the earliest aspect of Mesozoic, and even Palaeo- 

 zoic life, when their joint-heirs, the Palajo-Crinoidea, wei'e in existence; the Exocyclic 

 system, broken up posteriorly by the egress of the excretory opening, to Oolitic time; 

 and, among the Spatangida?, the Ethmophract system, of four costals, with the 1 and 

 4, and the radials I and V, or these last alone, closing from either side, is a badge 

 of seniority, as the other one, the Ethmolysic, with the restored costal 5 separating 

 the I and V, and the madreporite retrograding, is a sign of juniority. Future re- 

 searches Avill decide whether we should be right in seeing in the dissolving calyx of the 

 Pourtalesiaj an indication of a still later origin. 



These, I believe, are some of the leading features in the history of the calycinal 

 system, as one of the constituents of the Echinoidean skeleton. A large and powerful 

 structure, closely specialised for a function of fundamental importance in the ecpnomy 

 of some remote ancestral type, is inherited, in an early state, by a descendant in which, 

 from a total change in the mode of life, the very purpose no longer exists for which 

 it was originally contrived, and to which its parts were adapted. It long retains certain 

 marked features which even to this day reveal its origin, but — unlike its Crinoidean 

 sister- structure which, with functions unaltered, multiplies its components — it remains 

 simple as from the beginning, and, superfluous as it has become, gradually declines in 

 intrinsic vigour, and is given up to subserving activities that had no share in its pre- 

 vious existence. Invaded by contending organs and yielding to their various ten- 

 dencies, it has its parts deeply modified and even to some degree suppressed, and, al- 

 though still tru(! to its type, and asserting, so to say, its unimpaired independence hj 

 redintegrating its injured frame, it dwindles nevertheless from age to age in every 

 succeeding form, and is seen to fall into decay and dismemberment, and to lose one 

 by one its charactei-istics, till at last little remains of its original constitution. 



K. St. Vot.-Akad. lIiiuiU. Hil. l!l 



II 



