BIHANG TILL K. SV. VET.-AKAD. HAXDL. BAND 18. AFD. IV. N:0 I. 47 



disks, while the armature of the moutli serves tlie sole pur- 

 pose of grasping the food handed down to it or picked up on 

 the ground on which the animal leads its wanderiug life. The 

 whole of the maxillary system, homodont and in all its parts 

 of a compaot, solid tissue, constitutes a reversed cone, the vertex 

 of which is formed by the points of the teetli, the nearly erect 

 [)Osition and outward curvature of which eonibine to make 

 them a five-bladed powerful instrument adapted by their härd 

 and trenchant cuspidate crowns to biting and tearing to pieces 

 even iixed or else resisting objects, Corals, Balani, Serpulre; 

 g■o^'erned as they are by powerful muscles, the interpyramidals 

 which by contraction close their gape so as to give it a hrm 

 hold of whatever it seizes; the protractors, attached low, in- 

 serted high, thns capable of protruding it forcibly far beyond 

 the peristome; the lifting retractors attached high, inserted 

 low, assisting in opening the gape; and in order to secure to 

 the whole apparatns the greatest possible freedom of niove- 

 ment, a pliant buccal membrane has been brought about by 

 secondary reabsorption of legitimately preexisting parts, set 

 at work simultaneously with the act of dentition. 



I have elsewhere' given a description and figure, repeated 

 on next page, of the internal aspect of the peristomal region in 

 tlie Discoidea cylindrica, the low form, as described by Cotteau- 

 and AVright.3 The stoma is very slightly oval, the antero- 

 posterior diameter III — 5 being somewhat the longer. The 

 ambulaera project a little more than the broad interradia. 

 The auricles rise on the ambulaera and recline with their 

 basilar portions upon them as well as tipon the strongly swel- 

 ling columns of the interradia. Their upper parts are more 

 erect. The upper margin is thin, and presents at its ambula- 

 cral angle a projecting ear made distinct by a slight impres- 

 sion. Each auricular branch is connected with that of the 

 nearest ambnlacrum by a wall rising from the interradium, 

 nearly attaining the heiglit of the auricle, almost linear at its 

 upper margin and slightly hollow on the adoral surface. The 

 sutures which define the auricular l)ranches and the first in- 



' <Jn a reeent form of the Ecliiiiocouidjp. Bihaiig till K. Svenska Vet.- 

 Aka.l. Handlingar. XIII. IV, n:o lU. 



~ Palénntologie franeaise, VII, Pl. 1010, f. 1. 



' British fossil Ecliinodermata of tlie Cretaceous formations, pl. XLVI, f. 2. 



