JAWS. 



I\ the Desmosticha the jaws are composed of the pyramid, the brace, the 

 rotulae, the compass, and the teeth. 



The pyramid (PI. II". f. 1,2, if, -is, 49 -SI, 34-37) has an outer con- 

 cave side and two flattened sides, forming the hollow groove in which the 

 tooth runs. The pyramid is so placed that its vertex is near the actino- 

 stome. The pyramid is divided by a longitudinal suture, and it either lias a 

 solid face at ils upper extremity, as in the Cidaridae ( PI. II". f. /), or its 

 upper part is more or less open, occupied by a wide triangular foramen in 

 the other Desmosticha ( PI If"./. 15, 3d). The prolongation of the ahactiual 

 part of tin' lateral part of the foramen (its apophyses) either forms a closed 

 are in Echinidae proper, or an open one in dilferent families (Cidaris. Arlia- 

 ciadae, and Diadematidae). The lateral faces are lamellae laterally striated 

 (PI II". f. 17, •.'';). enclosing a cavity which occupies the greater part of the 

 abactinal part of the pyramid, the lower part of the pyramid alone forming 

 the solid guide for the tooth. This structure is best seen in I'l. 1 1 " . f. 34, m 

 which one of the lateral faces of the pyramids is removed. The brace is 

 placed between adjoining pyramids; it is a lengthened flat piece (PL II". 

 f. n. in. 28 ■:<'. 10 ;.') below the level of the compass, which is placed with 

 its convex side uppermost (PI. II". f. r/, .;/ .;.;, 4.3 -45, 57), the single 

 point towards the centre of the pyramid and the fork outward. The Lantern 

 of Aristotle, as the whole masticatory apparatus is called, is composed of 

 five pyramids, made up of two halves, the five braces, the five teeth, and 

 the five compasses, which, as shown by Meyer, arc made up of two pieces 

 soldered together. The teeth are composed of two parts, — one the tooth 

 itself; the other the upper soft part, the matrix of the tooth. The teeth of 

 the Desmosticha are either simply grooved, as in Cidaris and Diadema (PI II". 

 f. 12, 47), or else keeled in the middle, as in Arbaciadae and Echinidae {PI. 

 II". f. 26, 27, 59, 60). In the Clypeastroids the jaws have the same general 

 structure, only we find that the compass is wanting {PI. XI".). The true 

 structure of the jaws of Clypeastroids was early known ; Parra, in his Hist. 

 Nat. de Cuba, having already given a correct description of them. As far as I 



