KONGL. SV. VET. AKADEMIENS HANDL. BAND 19. N:0 7. 17 



analogy it seems probable that the continuous series of much reduced plates which 

 in the Collyrites connects the periproctal area with the dismerabered calycinal systern, 

 and keeps asunder the interradia 1 and 4, raay be found to consist of true interradials 

 belonging to the odd posterior interradium 5. 



It is iinportant to know whether the highly anomalous disposition of the 

 perisomatic system in Pourtalesia Jeffreysi is a peculiaritj^ more or less characteristic 

 of that species alone, or, like the frontal recess, a feature common to all the different 

 forms of the group. In Pourtalesia laguncula Al. Ag. ^), Pl. VI, fi(/. 37 — 40, the labrum, 

 5, 1, much larger than in P. Jeffreysi, excluded from the peristoine, expands posteriorly, 

 drives asunder the first plates of I and V, and attains the adoral ends oi 1 b 1 and 1 

 b 2+3, å a 1 and 4 a 2, herein very strongly differing from the labrum of that species. 

 But the interradia 1 and 4, expanded as in P. Jeffreysi, exhibit exactly the same 

 disposition by uniting mesially, and forming a continual broad ring passing round the 

 middle of the body, and where they meet dorsally from either side, their junction is 

 completed by the interposition of detached plates of the postei'ior odd interradium 5, 

 Pl. VII, jlg. 52. 



According to the figures given by Al. Agassiz ^) of Spatagocystis Challengeri, its 

 interradia 1 and 4 unite ventrally as well as dorsally, and by their interposition, and 

 that of the ambulacra I and V, the labrum is separated from the sternum, which is 

 very minute and narrow. Dorsally a row of plates of the interradium 5 separates 

 them from the calycinal system. 



These three forms, therefore, Pourtalesia Jeffreysi, P. laguncula and Spatagocystis 

 Challengeri, all agree in the annular disposition of the middle region of the perisome 

 formed by the lateral interradials, 1 and 4; — but they seem to be extreme cases. 



For, as far as the fragraentary condition of the specimens available has permitted 

 me to ascertain these points, Pourtalesia cainnata Al. Ag. ^) and P. ceratopyga Al. 

 Ag. *) as well as Echinocrepis cuneata Al. Ag. ") differ from them in a marked manner. 

 Like P. laguncula they all have the labrum, 5, 1, expanding aborally. In P. carinata, 

 Pl. VI, Jiff. 42, 43, 45, 46, as in that species, but unlike what is seen in P. Jeffreysi, 

 the arabulacrals I, 1 and V, 1 are bi-seriate, a and b, but aborally they are not, as in 

 P. laguncula, separated from I a and b 2, V b and a 2, but contiguous to them. 

 Consequently the interradials 1 and 4 do not join one another on the mesial line, 

 but are wholly lateral, as they ought to be. They do not, however, form part of the 

 peristome, being excluded from it by the close contiguity of the ambulacra I and II, V 

 and IV, but are to be seen, narrow, deformed and isolated, squeezed in betAveen the 

 I b 1 and II a i on the right, and the Y a 1 and IV 6 / on the left, as if forced out 

 of their proper places in the peristome. The fragment examined permits me to observe 

 how the V and IV unite behind the 4, /, and how the 4, 2 makes its appearance outside 

 the union of V a 5 and IV /; 2, fig. 42, 46, 45. On the right side, Avhere the 1, 7 is 



1) Chall. Rep. p. 137, pl. XXII a, fig. 7—15; XXXI, fig. 1—11. -) Ib. p. 141, pl. XXVI. XXVI a. 

 3) Ib. p. 133, pl. XXVIII a. -») Ib. p. 134. pl. XXVm, XXXV h, fig. 17. ■') Ib. p. 143, pl. XXVII, 

 XXXV a, fig. 9—13. 



3 



K. Vet. Akad. Handl. Baud 19. N:o 7. 



