Prof. J. Muller on the Structure of the Echinoderms. 119 



physes are very depressed in correspondence with the flattening 

 of the alveoli. Short thin muscles pass from the auriculae to the 

 under surface of the alveoli. 



The teeth of the Clypeasters, which are fixed in the groove of 

 the alveoli, are only naked at their outermost extremity, the rest 

 being covered by a proper soft membrane which must be re- 

 garded as the sac-like matrix of the tooth. 



In describing and figuring the teeth of a Galerites (Mem. 

 Geol. Survey, Lond. Decade 3. pi. 8) Forbes expresses the opi- 

 nion, that perhaps all Cassidulida have teeth. I have examined 

 a specimen of Echinoneus which still retained its buccal and anal 

 plates, and although dry had lost nothing from its interior. 

 However I could find no teeth in the contained matters, which 

 consisted only of coarse sand, small Grasteropod shells, and frag- 

 ments of shells, such as we meet with in sea-sand, and which 

 proceeded from the contents of the intestine. 



The stellate gap in the skeleton above the mouth of the 

 Ophiurida and Asterida is well known not to be the mouth, but 

 its antechamber. The mouth itself is round and lies deeper in 

 a membranous diaphragm. The anterior chamber is therefore 

 comparable to the vestibule in front of the mouth of the Holo- 

 thuriada. 



In the Ophiurida the stellate gap above the membranous dia- 

 phragm is surrounded by twenty pieces, which are simply the 

 most anterior ambulacral plates united with five pair of inter- 

 ambulacral plates. The anterior ambulacral plates are in pairs 

 like all the others ; they are as usual united A;\dth the foUomng 

 ambulacral plates by muscles and articulations, but their union 

 with one another takes place not by sutui-e but by a toothed 

 joint, and is therefore moveable. These anterior ambulacral 

 plates bound the open angles of the oral gap, while the inter-am- 

 bulacral pieces correspond with its salient angles ; the ambulacral 

 plate is united with the inter-ambulacral plate of the angle of 

 the mouth by a firm suture. The union of the inter-ambulacral 

 plates constituting any one of the salient angles of the mouth 

 takes place by a denticulation, which allows of motion by means 

 of transverse muscles which approximate the crura of the open 

 angle and unite the anterior ambulacral plates of two ambulacra. 

 The external edges of the angles of the mouth are beset with cal- 

 careous papillae towards the oral clefts — papilla marginales — mar- 

 ginal papillae of the oral cleft. Upon the vertical edge of the 

 oral angle again, we find in many genera a multitude of papillae, 

 the papilla angulares or papillae of the oral angles (dental pa- 

 pillae, Miiller and Troschel); below these in the Ophiuridce stand 

 the dentiform labial plates, arranged in a vertical series, and 

 which I denominate pala angulares instead of teeth. Are these 



