THE PERISTOME. 81 



Text-figs. 40-54. — Characters of peristome and base of the corona in repre.sentativc Ecliini. 



40. Jiolliiiocklaris tirchiiica sp. nov. Ord()\ician, Russia. From Plate 1, fig. 1. On the peristome two rows of ambu- 

 lacral plates. 



41. t'lioniKixdiiKi iiliiccnlaWyvWU' TliomMm. Young. Diani. 9 nun. Adapted from A. Aga.ssiz, 1004, Plate 43, fig. 3. 

 On the ])eri.slomc' two rows of ambulaeral plates. 



42. The same. Diam. 7 mm. .\dapted from A. Agassiz, 1904, Plato 43, fig. 1. Peristomal jilates with pores in 

 a central iierijiodium. 



43. The same, adult . Off Cape May to Cape Sable, 956 fat h. Diani. 50 mm. R. T. J. Coll., 707. X 3. On the 

 peristome many row's of ambidaeral plates. 



44. The same. Peristomal gills enlarged. 



45. (•oniocidaris canalicitlala A. Agassiz. Young. Diam. 1.45 mm. From Plate 2, fig. 1. On the peristome one 

 row of ambulaeral plates. 



40. Eiicidari.-i Irihidoiihs (hamiirck). Bahamas. Diam. 45 mm. R. T. J. Coll., 708. X 3. On tlie jteristome many 

 rows of ambuhirnd ami in addition intcrradial non-a,inl)ulacral plates (compare with young, Plate 2, hg. 0). 



47. Airhiicucidaris wnrlkini Hall. Lower Carboniferous. From Plate 9, fig. 0. Partially' restore<l. On the peri- 

 stome many rows of ambulaeral and in addition intcrradial non-ambulacral plates. 



48. MiioHccliiiuis iiiulliporus (Norwood and Owen). Lower Carboniferous. Restored. From Plate 50, figs. 7, 8. 

 On the peristome many rows of ambulaeral and in addition two rows of interrailial non-ambulacral ])lates; ambulacrals 

 pass from two plates orally to many on the iierijihery of ])eristome in each area. 



49. Slriingijlocnilnihix didbachiensia (O. F. Miiller). Y'oung. Diam. 1.2 mm. From Plate 3, fig. 11. On the peri- 

 stome one row of ambulaeral plates. 



50. The same. Y'ork Harbor, Maine. Adult. Diam. 40 mm. R. T. J. Coll., 709. X 3. On the jieristome one 

 row- of ambulaeral and scattered, small, non-ambulacral j)lates. 



51. Dentialodiadeina anidlaruiii A. Aga.ssiz. West Indies, 955 fath. Diam. 9 mm. R. T. ,J. Coll., 670. X 8. On 

 the peristome one row of large ambulaeral plates. 



52. Echinamchnius panna (lAimiirck). Ea.stport. Maine. R. T. J. Coll., 747. X 2. On the peristome no plates. 



53. 'Vhe, same. X 6. PerislouK^ enlarged. 



51. Kchiiiocardiuni flavescens (Miiller). Adapted from Loven, 1874, Plate 3, fig. 34. On the peristome many non- 

 ambulacral plates only. 



In text-figures 40, 41, 43, 45, 49, 52, and 54 the {irimordial interambulacral plates are in place in the basicoronal row; 

 in other figures they have been resorbed, with or without additional plates. 



two rows of ambulaeral plates arouutl the mouth instead of one row, a difference which is 

 bridged in some types as shown in the development of Phormosoma. 



A second type of peristome is that in which the area is plated with several rows of ambulaeral 

 plates. The primordial ambulaeral plates surround the mouth as usual, and succeeding rows 

 are composed of ten plates each, which are continuous, as are the primordial plates. Such a 

 condition is seen in Hyattechinus (Plate 23, fig. 1), Pholidechinus (Plate 28, fig. 1), Lepidesthes 

 (Plate 68, fig. 3), and Palaeodiscus (Plate 18, fig. 2). It is to be observed that in all these 

 types the primordial interambulacral plates were doubtless in place in the basicoronal row of 

 the corona. The same character of f)nly amlnilacral plates on the peristome is seen in Recent 

 Asthenosoma (Loven, 1892) and Phormo.soma (text-fig. 43). It also occurs exceptionally in 

 cidarids, as shown by Mr. Agassiz (190-1, p. 30, Plate 11, fig. 1). In the Cidaridae, however, 

 the primordial interambulacral plates are not retained at the base of the corona, but have been 



