70 



ROBERT TRACY JACKSON ON ECHINI. 



cidaris (text-fig. 22), which continues to build a single column. This same character of a single 

 plate ventrally, but succeeded by two plates in the second row, is characteristic of the young 

 of all modern regular Echini, as shown by Loven (1892) (Goniocidaris, text-fig. 23). In the 

 adult of most regular Echini the single plate and probably more have been resorbed by the 

 advance of the peristome (Eucidaris, text-fig. 24). In the Palaeechinidae with many columns of 

 plates, apparently only one plate has been resorbed, when we find two plates in the basicoronal 



27 



28 



29 



30 



31 



Text-figs. 22-31. — Characters of the base of the interambulacrum in representative Echini. 



22. Bolhriocidaris archaica sp. nov. Ordovician. From Plate 1, fig. 1. 



23. Goniocidaris canaliculala A. Agassiz. Young. From Plate 2, figs. 1, 2. , 



24. Eucidaris tribuloidcs (Lamarck). Bahamas. 



25. Melonechinus muUiporus (Norwood and Owen). Lower Carboniferous. From Plate 57. 



26. Archaeocidaris wortheni Hall. Lower Carboniferous. From Plate 9, fig. 6. 



27. Plexechinus cinclus A. Agassiz. Adapted from A. Agassiz, 1904, Plate 58, fig. 1. 



28. Echinocyamus -pusiUxis (Miiller). Adapted from Lov6n, 1874, Plate 44. 



29. Rotula dentata (Lamarck). Adapted from Lovfn, 1874, Plate 46. 



30. Pcrischodomus hiscrialis M'Coy. Lower Carboniferous. From Plate 64, fig. 2. 



31. Tiarcchinus princeps (Laube). Triassic. Adapted from Loven, 1883, Plate 13, figs. 152, 154. 



In figures 22, 23, 27-31 the primordial interambulacral plate is in the basicoronal row; in 24-26, it, with or without 

 additional plates, has been resorbed. 



row (Melonechinus, text-fig. 25), or in the Archaeocidaridae, several rows of plates may have 

 been resorbed, and we find four plates in the basicoronal row (Archaeocidaris, text-fig. 26). 



In modern Echini where there has been no resorption, the single primordial plate is retained 

 in the adult. This character, as shown by Loven (1874) and others, occurs in most clypeas- 



