18 S. LOVEN, ON POURTALESIA, A GENUS OF ECHINOIDEA. 



considei*ably larger, the broken test shows only the junction, behind it, of I b and II a. 

 The labruin, like that of P. laguncula, is very iiarrow adorally, jig. 42 — 44, — it is härd 

 to say whether it reaches the peristome or not, Jig. 46, — and enlarges aborally, till it ineets 

 a pair of plates that occupy exactly the place between the ambulacra I and V, regularly 

 allotted to the sternum, 5 a b 2. According to Al. Agaösiz they seem to carry spines 

 bigger than others, on closely packed tubercles. From all this it follows that in P. 

 carinata the interradials 1 and 4 do not join ventrally in the niiddle, and conse- 

 quently do not form a continuous ring. Dorsally they are very much the same as 

 those of P. lao'uncula. 



o 



In Pourtalesia ceratopyga, Pl. VII, jig. 48, 49, 50, with its bi-seriate ambulacrals 

 I and V, there also appears between them and II and IV, on either side, a wedge- 

 shaped interradial 1, 1 and 4, 1, excluded from the peristome, but on the left side, 4, 1, 

 not very far from being admitted to its legitirnate position in it. Dorsally, /i^. 51, the 

 interradials 1 and 4 unite, broadly separating the last plates of 5, out of which a few 

 are pushed forward so as to infringe upon the calycinal system. 



Echinocrepis cuneata Al. Ag. O ventrally seems to present the same disposition 

 Pl. VII, fig. 53. The first plates of 1 and 4, excluded from the peristome, occupy the 

 same places as in the two preceding species, and probably are contiguous to their re- 

 spective interradia. The labrum, 5, perhaps without reaching the peristome, is produced 

 aborally, and according to the figure given by x\l. Agassiz, seems to reach the ambu- 

 acrals I a b 2 and Y b a 2, which separate it widely from the sternum, 5, 2. Dor- 

 sally, fig. 54, the 1 and 4 laterally touch the calycinal system, while posteriorly the 

 ambulacra I and V intervene between it and the last plates of 5. 



Thus, Avithin the definite and narrow limits of the little group of the Pourtalesiada^, 

 consisting at present of only ten species distributed into three genera, and held firmly 

 together by a few essential and constant characteristics, there exists in the perisomatic 

 system a movement at once forcible and anomalous, tending to transform its most im- 

 portant elements into something unlike every precedent. Ventrally the change may 

 be supposed to begin with the withdrawal of the 4, 1 and 1, 1 from the peristome, 

 and their secluded reception between I b and II a, V a and IV b, as in Pourtalesia 

 carinata, P. ceratopyga and Echinocrepis cuneata, while these same ambulacrals are left 

 in legitimate contiguity to the respective I a b 2 and Y a b 2; — and to end, in 

 Pourtalesia Jeffreysi, P. laguncula and, apparently, in Spatagocystis Challengeri, with 

 the entire removal of 1 and 4 from their old places between I and II, V and IV, to 

 a new position, on the mesial line of the skeleton, where they interrupt the succession 

 of plates in the ambulacra I and V and the interradium 5. Dorsally, while in Echinocrepis 

 the 1 and 4 can hardly be said to join one another, they in all the other species exa- 

 mined unite freely and largely, with the intervenience of plates from the interradium 5. 

 And thus in these three species at least, the interradia 1 and 4 coinbine to form a 

 continuous ring all around the raiddle of the body. Once before, early in Mesozoic 

 time, for a Avhile and not unlike a trial soon given up, a structure resembling 



>) Chall. Rep. p, 143, pl. XXXV a, fig. 10. 



