40 SVEN LOVEN, ECHINOLOGICA. 



coiistaiit fluctuatioiis of ab.sorptioii and renewal mutually main- 

 taining their iirm conneetioii, the auricles iiisensibly gliding 

 aborally, while the ambulacral margin retreats, tluis liolding 

 their positions as supports of the maxillary system. It was 

 from earlier observations on this snbject that I formerly was 

 led to coneeive the auricles as belonging morphologically to the 

 dental system.' The phases of their evolution here described, 

 though not altogether inconsistent witli this supposition, can- 

 not be held as sufficient evidence in its favour. When the 

 structure and evolution of all these parts shall be better known 

 thau they are at present, this (juestion, with many others, 

 will arrive at its decision. 



Dissimilar as are the Ectoibranchiates with respect to these 

 parts, they come near to the Cidarid(u in the structure of the 

 interradia. The peristomal plates of these grow upward into 

 internal processes, which tliicken and form a wall or a strong 

 ridge towards the buccal membrane. 



The structure of the dental apparatus in the Regularia 

 has long been known in its principal points, thanks to many 

 able investigators, among which Valentin- and T. H. Stew- 

 ART-'^ stånd foremost, while the histology of the teeth has been 

 carefully studied by Giesbrecht.* In eaeh of the five equal 

 and uniform pyramids (w^oodcuts p. 53, iig. 1 — 4) the two la- 

 teral surfaces, b, are flat and densely marked with transverse, 

 line, feebly undulated, parallel ridges for the attachment of the 

 inter-pyramidal muscles. The wings converge inwards, but 

 leave an open space between their pectinated cesophageal mar- 

 gins. The third, external. triangulär surface, a, arcuated, convex 

 and smooth, emarginated in its upper part by the »foramen 

 magnum» of Valentin, presents outwardly, within its narrow 

 lateral margins two longitudinal depressions, and a mesial 

 elevation which expands adorally and forms alone the whole 

 of the adoral termination, taking there, the shape of a spoon 

 with a small notch at the tip, the processus labialis. A me- 

 sial suture, ahvays present, marks the symphysis of the two 



' Etudes, p. 19, 2S, 31. 



- Agassiz, Monographies cVEchinodermes I. 



3 Proc. Zoologieal Society, 1861, p. 58, Pl. X, XI. 



■* Morpliologisohes Jahrbuch VI. 



