KONGL. SV. VET. AKADEMIENS HANDLINGAH. BAND. 19. N:0 7. 13 



When from this eminently archaic form we turn at once to Pourtalesia Jeflfreysi, 

 so profoundly contrasting in every particular of its build, the live interradia are found 

 to conform to the rule prevailing in the Neoiiomous Echinoids, of Mesozoic and later 

 origin: they all consist of a double series of plates, PL H, fi'J- 9. The two anterior, 

 2 and 3, considerably smaller than the lateral ones, 1 and 4, are, as usual, enclosed 

 between the frontal ambulacrum. III, and the paired ambulacra II and IV. Occupying 

 the corners of the front, with their inner series of plates drawn in into the infra- 

 frontal recess along with the ambulacrum III, they rise on either side, and terminate 

 dorsally within the angles formed by the trivious ambulacra and the calycinal system. 

 Their plates are, upon the whole, smaller than any other in the whole test. They enter 

 the peristome each with a single plate 2 i 7, 3 n U PL 11, fig. 9; HI, 12; IV, 

 18, 19, belonging to their inner series, a long, rather narrow and flexuous plate, 

 joining anteriorly the a 1 and 6 i of the ambulacrum III, extending along the nearly 

 vertical sides of the stoma, and meeting ventrally and posteriorly the minute labrum 

 marked 5, PL II, fig. 9; IV, 16, thus excluding from the peristome the ambulacra 

 II and IV, I and V. In the same series 2 h and 3 a, this first plate is followed by 

 a large second plate, joining its entire aboral margin, and preceding a series of ten or 

 eleven sub-hexagonal oblong plates, soon diminished in size, and ending at the calycinal 

 system Avith a small squarish or sub-pentagonal plate, PL II, fig. 9 ; V, 25, 26, 27, 

 28. The outer series, 2 a and 3 b, PL II, fig. 9; III, 12; IV, 18, 19, of ten plates, 

 commences with a middle-sized plate, 2 a i and 3 b 1, having its hinder extremity 

 inclosed between the ambulacrals II b 1, 2, o, and IV a, 1, 2, 3, and the interradials 

 2 h, 1, 2, 3, and 3 a, 1, 2, 3, and thereby excluded from the peristome. It is followed 

 by nine hexagonal plates, somewhat larger than those of 2 b and 3 a, increasing up 

 to the fourth or hfth, then diminishing, and joining the calicynal system with an 

 irregularly squarish terminal plate, PL V, fig. 2.5, 26, 27, 28. 



The lateral interradia 1 and 4 are much more expanded, as generally in the 

 Spatangidse, and especially in the Prymnodesmic genera. They constitute the flanks of 

 the body. On the dorsal surface their plates are the largest of all, and on the ventral 

 inferior to the sternum, 5, 2, alone. By an arrangement hitherto without parallel, they 

 commence, not, as by rule they ought, in the peristome and separately, but apart from it 

 and jointly, in the middle of the ventral surface, PL I, fig. 2; II, 9; III, 10. As in 

 most SpatangidaB, Prymnadete and Prymnodesmic, the first plate of these interradia, 

 though properly belonging in common to both series 1 b and 1 a, 4 b and 4 a, still 

 seems to appertain more especially to the anterior of them, 1 b and 4 a, ^) so also in 

 Pourtalesia Jeflfreysi it is the first plates of the anterior series that alone are reci- 

 procally contiguous, and this along the entire inner margins of their first plates, 1 /, 4 7, 

 thus separating from one another the first and second plates of the ambulacra I and V. 

 In the specimens from which the figures PL I, fig. 2, 8; II, 9; III, 10, were taken, 

 the first plate of the interradium 4 is twice the size of 1, /, and alone in contact with 

 the ambulacrals I, / and V, /. But in this respect there may be some variation. These 



») Etudes, pi. XXVIII— XXXVIII, XL— XLIII. 



