KONGL. SV. VET. AKADEMIENS HANDLINGAU. BAND. 19. N:0 7- 29 



alone maintains its normal share in the coraposition of the peristome, while the two 

 paired trivious ambulacra, diminished in size, and the bivious ambulacra, reduced to 

 single plates only, ai'e all excluded from it, and so closely pressed togethei* on its 

 under side, as to occupy, all taken together, considerably less space than the front 

 ambulacrum alone on the upper. The space intervening between the minute inter- 

 radial 5, 1, and the large ambulacrals III a 1 and III h i, is filled by the interradials 

 1 h 1 and 3 a i, Pl. II, jig. 9; III, 12; IV, 18, 19, and, as these last are very high, 

 the stoma assumes an elliptical form, unseen among the rest of the Echinoidea. P^or, if 

 in Anancites and its allies the labrum is but slightly developed, and even less so in 

 Collyrites whose stoma is longitudinally sub-elliptical, the constituents of the pei'i- 

 stoma are nevertheless normal everywhere. In Pourtalesia Jeffreysi, on the contrary, 

 it has been seen that they are: ambulacrum III, interradials 2 b 1 and 3 a 1, and in- 

 terradial 5, while the ambulacrals I, II, IV, V, interradials 1 a 1 and 3 h 1, and inter- 

 radials 1 and 4 are excluded. The plane of the peristoma thus composed is nearly verti- 

 cal, and perpendicular to that of the longitudinal axis of the body, or somewhat over- 

 hanging into the peritoneal cavity; it is not, as in the Spatangidse in general, all but coin- 

 cident with the stomato-proctic axis, but, as in the Endocyclic forms, though in an 

 opposite nianner, perpendicular to that also. In one of the specimens the two plates 

 1 of the ambulacrum III, and the interradials 2 b 1 and 3 a 1, in a part of their 

 margins are dissolved into an assemblage of minute rounded laminse, Jig. 19, 20, but 

 in two other specimens they are entire. 



The buccal membrane, PL IV, jig. 20, is destitute of calcified deposits. In its middle 

 is seen the oesophageal opening, an oblong slit, with naked, rather tumid and coarsely 

 wrinkled margins. Its direction is that of the vertical longitudinal plane of the body, per- 

 pendicular to the direction of the transverse fissure in the great majority of the Spatangida3. 



This description of the peristome and adjoining parts in Pourtalesia Jeffreysi 

 applies upon the whole equally to the same parts in the other species of the group; 

 the difierences presented by the ambulacra I, V, II, IV, will be taken up hereafter. 

 The species all agree in the important points resulting in that most striking feature, 

 the deep anterior recess. There can be no doubt that the elliptical frame surround- 

 ing the stoma in the Pourtalesiadte, widely different though it be in its composition, 

 is strictly homologous to the peristome in all the other Echinoidea, and, no less so, 

 that the vertically placed membrane with its longitudinal slit is homologous to the 

 horizontally stretched membrane in the Dentifera and Edentata, with the rounded or 

 transverse orifice of the alimentary canal. But the peculiar strncture of these parts 

 in the Pourtalesiadae is such as to call forth the idea, that what is going on here may 

 be looked upon as the first move, so to speak, towards forming a rudimental mouth, 

 a cavuiii ovis, the iuvaginated parts of which, if they were flexible and provided wqth 

 muscles, might be protruded like a proboscis, which, owing to the great size 

 of the frontal ambulacrals and interradials, would be directed downwards and back- 

 wards. As it is, its rigid and motionless lips are indicated by the line of in- 

 curvation all around its entrance, its palate, sit venia verbo, Pl. Ill, Hg. 10, 

 12; IV, 18, 19; VI, 45, is made up of the large and unreduced bi-seriate plates of 



