28 8. LOVBN, ON roUUTALESlA, A GENUS OF ECHINOIDEA. 



peristoriKj, and made up of a thicker calcified tissue of coarser meshes, and two narrow, 

 more finely reticular, lateral, perhaps slightly movable pieces. ') 



When any of these various characteristics of the Echinoidean peristome is looked 

 for in Pourtalesia Jetfreysi, it at once becomes obvious, that the exceptional case of 

 Palaiostoma no less than whatever there is in the others of conformity to a general 

 law, is set aside in the disposition of the corresponding parts of its skeleton. Its 

 peristome is altogether of a peculiar construction, PL II, jig. 9; 111, 12; IV, 15 — 19. 

 Out of the five ambulacra one alone, the front ambulacrum. III, with its greater part 

 sharing in the singular incurvation of the whole region, partakes of its formation, the other 

 four are all excluded from it, and, while the front ambulacrum and the tAvo lateral ones, 

 II and IV, each begin with a pair of plates, the two bivious ambulacra each commence 

 with a single plate only. Now it is known that, while in the Archajonoraous Echinoidea 

 the peristomal plates generally maintain their entire breadth,^) in the Neonomous, and 

 more particularly in the Spatangida3 ^) they are more or less contracted adorally, especially 

 in the biviuin, and more so in the young than in the adult. In Pourtalesia Jett'reysi 

 this feature is carried to an extreme. The front ambulacrum. III, enters the peristome 

 with its entire breadth, while the ])eristomal plates of the bivium and those of the 

 two paired ambulacra of the trivium, II and IV, are all diminished, cuneiform, nar- 

 rowing adorally almost to a point. The two pairs that belong to II and IV are very 

 small, half the size of the single ones of the bivious ambulacra, I and V, and all these 

 six plates together have their adoral contracted portions bent over abruptly, at an 

 acute angle, into the hollow of the infra-frontal niche, at the bottom of its ventral 

 sinus, tlius forming there a keen edge, strengthened inside by strong partitions, /7,9. 17, 

 reminding of the trabecula? of the Clypeastridto. In consequence of this incurvation, 

 while the greater part of their surfaces is visible in the ventral aspect, fig. 2, lo, their 

 adoral terminations are not visible, unless their portion of the test be cut out, and 

 placed with its inside toward the observer, fig. 16. Then it may be seen, although not 

 without some difficulty, that the narrowing terminations of the two single first plates 

 of the two liivious ambulacra, I and V, as well as the plates II a 1 and IV b 1, of 

 the two paired trivious ambulacra, do not reach the margin of the peristome, but are 

 excluded from it by a single mesial subtriangular plate, 5, which is the labrum, and, 

 as such, belongs to the interradial system, while the terminations of II 6 i and IV 

 a 1 are not even visible in that position, being situated on the curvature itself and 

 thereby concealed. Thus, in this extraordinary Echinoid, the front ambulacrum, III^ 



') See S. Loven; Otn Leskia rairabilis Gray, Ofversigt af K. Vetenskaps-Akaderaiens Foi-haiidliiigai-, 1867, 

 p. 4.31. In that paper I regarded the five-valved covering of the stoma of Paheostoma as homologous to the 

 "Pyramid" of Echiiiosphiera and Sphitroiiis. I soon afterwards perceived tiiat this was an error, as Lutken 

 clearly set forth in his memoir: Endnii ett par ord om de ganile Sbliliers wSnabclu og Miuul, Videnskablige 

 Meddelelser fra den Naturhistoriske Porening i Kjobenhavn, 1869, 160; Geological Magazine, V, 179; Canadian 

 Naturalist, new series. III, 437. 



2) Etudes, pi. X, tig. 84, 86, 89, 91; pi. XVII, tig. 140 — 148; pi. XVIII, tig, 153—158; pi. XIX, fig. 

 165; pi. XX, fig, 166. 



») Etudes, pi. Ill, fig. 32—35, 39; pi. IV, fig. 41—43; pi. V, fig. 46-48, 51, 54; pi. VI, fig. 55, 

 58—60; pi. Vll, fig. 61—64, 66, 67; pi. XXII— XLIII. 



