KONGL. RV. VKT. AKADEMIENS IIANPL. RAND 19. N:0 2. 15 



containing- the matter of two plates, is so out of proportion to the plates 4 a 2 and 

 4 a 5 taken together, which it represents, but this is often the case also in the Spa- 

 tangidaj, and, as in these so in Pourtalesia JefitVeysi, one or two among the following 

 plates in 1 6 to some extent compensate for the defect. And finally the position of 

 the heteronomy in these very same plates clearly testifies, that the tAvo remaining 

 innermost plates of the series are really the 1, 1 and 4, 1, and that thus these inter- 

 vadia, thougli displaced, are complete and normally constituted. 



The formulae of the heteronomy in the different groups are: 



Adetes: Ananciticlie 1 a 2 + b 2 ^ A a 2 : 4 !> 2. 



Prymnadetes : Desoria, Faorina ._ _.. 1 a 2 + b 2 ^ 4: a 2 : 4 /< 2. 



» Herniaster — Moira .._ 1 « 2 + a .v = 4 6 2 : 4 A .?. 



Prymnodesmians: all the genera 1 a 2 + a 3 = 4 b 2 : 4 h 3. 



Pourtalesia: two species 1 b2 + b3 = 4a2: 4a3. 



The union, in 1, of two plates into one, which in the great majority of the Spa- 

 tangida? takes place M'ithin the a series, and in the Adetes and a few Prymnadetes 

 within both series, a and b, in the Pourtalesia^ is transferred, entirely, to the b series, 

 a deviation corresponding with their remoteness in other respects. Everywhere the 

 heteronomy is, on the right, confined to the interradium 1. Evidently connected with 

 the obliquity represented by the axis fio)^ it is derived from some point in the larval 

 development not yet understood, a mark, perhaps, recording the heterologous position 

 of the young Echinus in the interior of its Pluteus. *) 



In ascending the tumid flanks of the body, the interradia 1 and 4 in Pourtalesia 

 Jeft'reysi slightly' incline forward, and terminate in joining again from either side on 

 the back, nearly at the anterior third of the entire length, PI. 1, fig. 1; III, 11; V, 

 25 — 28. There they end, without attaining their proper position, as by rule they ought, 

 in contact with the calycinal system, being kept apart from it by the intervenience 

 of the terminal plates of the odd interradium 5. Thus, while ventrally they do not, 

 as in all other Echinoidea, take their origin separately, in the peristome, between the 

 ambulacra I and II on tlie riglit, and IV and V on the left, so dorsally they do not 

 terminate, as conformable to rule, isolated from one another, and at the calycinal 

 system, l)ut at a distance from it and uniting reciprocally. Below, they break through 

 the bivious ambulacra; above, they disjoin the posterior odd interradium and take up 

 among them its detached plates, and all through the space thus opened, they describe 

 a broad perisomal belt continuously encircling the body around its middle. 



Unlike the paired interradia just described, the odd posterior interradium is 

 represented, conformably to the general rule, in the peristome of Poui'talesia Jeffreysi, 

 by a true, though very minute labrura, 5, PI. 11, fig. 9; IV, 16. Its position is entirely 

 within the incurved forepart of the test, and there it is wedged in between the first 

 plates of the interradia 2 and 3 and the first of the ambulacra II, IV, and I, V. By 

 the interposition of all these plates the labrum is widely separated from the seconds 

 of 5, which here, as in the Spatangid*, are formed into a sternum, 5, a h 2, PI. /, 



') See Etudes, p. ;37— .39, pi. XVII, li<r. 140. 



