jaws. 689 



have examined the jaws of the Clypeastroids I find they differ mainly, as 

 Midler has already shown, in their manner of articulating irpon the auricles. 

 The rotulae are placed upon the sutures of adjoining pyramids (Pi. XI". f. 

 i h g), and are made up of three minute plates. The greatest variation in the 

 shape of the jaws consists in their great height, as in Clypeaster and Echi- 

 nanthus (PL XP'.f. 1-8), or in their extreme flatness, as in the Scutellae 

 proper and in Eehinodiscus (PI XI". f. 9 -is). The teeth are placed in a 

 groove of the pyramid, as in the Desmosticha. Muller is of the opinion 

 that it is to the rotulas of the Clypeastroids that we must compare the cal- 

 careous ring of the Holothuriae, as it is near these rotulse that the ambu- 

 lacral canals open outwardly ; the Holothuriae thus would want the pyramids 

 and the teeth: it also follows, from Midler's careful analysis of the mouth 

 papilla? of Ophiurans, that they are in no way homologous to the jaws of 

 Echini. 



The pyramids of Clypeastroids are triangular, flattened ; the outer side, 

 however, forming a re-entering angle (PL XI".), so that the junction of ad- 

 joining pyramids forms the apex of the pentagon of the jaws, and there 

 the rotulge are placed ; — see the figs, of Eehinodiscus auritus, seen in pro- 

 file and from above (PL XI a .f. 9, 12), which show the position of the rotulae. 

 The mode of articulation of the jaws upon the auricles is entirely distinct 

 in the Clypeastroids and in the Desmosticha ; in the Clypeastroids the auri- 

 cles are disconnected (Pi. XP . f. 4 ; PL XII". f. 4; Pi. XIIP. f. 5 ; PL 

 XIIP. f. ; PL XIIP. f. 3; PL XIIP. f. 1), and when the jaws are in 

 place they completely hide the auricles upon which they ride, as in PL XP. 

 f.4; PL XIP.f. 3,4; PL XXVIII f. 1,2; PL XXIX. lower fig.; PL 

 XXXIII. f. 3. In the Desmosticha, on the other hand, the jaws are placed 

 entirely within the line of the auricles (PI. XXVII. f. 1,5), from which 

 they are supported by a very complicated set of muscular bands, extending 

 in pairs from the sides of the auricles, from their base and from the inter- 

 vening spaces, to different points of the pyramid and of the braces ; thus the 

 jaws of all Desmosticha are capable of considerable motion. The greater 

 part of the jaws is, as is well known, surrounded by the membrane of the 

 lantern (Pi. XXVII f. 1,5); this membrane is in the Clypeastroids reduced 

 to the small bursiform appendages of the inner part of the pyramid. The 

 muscular system of the jaws of Clypeastroids is reduced to a very feeble 

 band attached to the under side of the pyramids and extending to the 

 auricles. 



