Vol. 55.] SILURIAN ECniNOIDE\ AND OPHIUROIDEA.. 713 



as we hiive learnt from Palceodiscus, made their appearance very 

 early in the xjhyletic development of the Echinoids. They occupy 

 in the young specimen the same position as in the adult, and, as 

 in Palceodiscus^ lie one above the other in the radial plane. It 

 is difficult to discover homologies for these parts, but since they 

 lie above the water- vascular canal thej^ may be assigned to the 

 Asteroid and not to the Echinoid system of the ambulacrum. In 

 ancient and primitive star-fishes the ambulacral plates are said to be 

 alternately, not oppositely, disposed ; and if this be the case, the 

 su2:gestion arises that the radius and rotula may correspond to a 

 pair of ambulacral ossicles, which have been pushed one over 

 the other in the course of development. The elongation and diver- 

 gence of the Echinoid ambulacral plates to form the epiphyses 

 would carry the Asteroid ambulacral ossicles in its course, and 

 during this translation a displacement might conceivably arise. 

 Such a view, however, is difficult to harmonize with the symmetrical 

 arangement of the muscles of the radii. The radius indeed, with 

 its forked end and connexion with the interambulacra, has the 

 appearance of representing an entire ambulacral arch. The rotula is 

 certainly a single and unpaired element, it arises as a single rudi- 

 ment, and sections through it show no trace of suture, while the 

 calcite which forms its skeleton is optically a single crystal. Its 

 position in the radius corresponds to that of the rudiment of the 

 tooth in the interradius, and it braces together the pyramids in 

 much the same fashion as the odontophore links the jaws of star-fishes. 

 Can there have been ten unpaired elements in the mouth of the 

 ancestral Asteroid ? 



A later stage of development than that represented by the young 

 Goniocidaris is furnished by a young example of Strom/ylocentrotus 

 drcehaJcetisis, 0. F. M., the mouth-parts of which have also been 

 illustrated and described by Loven. In this, as Loven points out, 

 the hemipyramids have not yet become closely approximated in 

 the radial plane, and consequently the interpyramidal muscles are 

 of considerable length. The epiphyses, as seen from within the 

 test, have acquired an interradial position, those of adjacent 

 ambulacra meeting along an interradius. This is the arrangement 

 of the parts in an Asteroid carried to an extreme. 



The lantern of Aristotle is at present in existing urchins dis- 

 tinguished by its great relative height, but if it originated from 

 ambulacral ossicles it must have been primitively far more depressed. 

 The upward growth, which we imagine to have taken place, finds 

 an exact parallel in the case of brittle-stars and star-fishes, the jaws 

 in both of these groups being extremely depressed in fossil forms 

 of Lower Ludlow age : indeed, it would not be far from the truth 

 to affirm that, as the thickness of the jaws in recent Ophiuroids is 

 to that of Silurian forms, so is the height of the modern lantern of 

 Aristotle to that of PalceocUscus. The proportion is not exact, but 

 nearly so. The rotulae in Palceodiscus lie at about the level of the 

 inner ambulacral series. 



The severance of the lantern from the plates of the test is an 

 Q. J. G. S. No. 220. 3 a 



