THE PEHISTOMi: gj 



TEXT-FIQS. 40-54. Characters of peristome anil ba.H<- of the .-on-.m in rrprem.t alive Ki-hini. 



40. Hothriocidarisarchaicasp.iiov. Ordovician, Uutwiu. From Hair 1, fig. 1. On il,,- i*n*tome two row.of.mbu- 



lacral plates. 



41. Phormosoma placenta Wyvillc Thomson. Young. Dium. mm. Adapt.-.! from A. Aganmi, 1904, IMaU- 43, fi K . 3. 



On the peristome two rows of aiiibulacral plates. 



42. The same. Diam. 7 mm. Adapted from A. Agassiz, lJO-f, Plat,. I.I, fig. I. PoriMtomnl plaint with porai in 

 a central peripodium. 



43. The same, adult. Off Cape May to Cape Sable, 95ti fall,. Diam. 5ii mm. It. T. .1 Coll , 707. X 3. On the 

 peristome many rows of ambulacra! plates. 



44. The same. Peristomal gills enlarged. 



45. Goniocidaris canaliculata A. Agassiz. Young. Diam. 1.45 mm. From Plate 2, fig. 1. On il,.. i- rutomc one 

 row of ambulaeral plates. 



46. Eucidaris tribuloitles (Lamarck). Bahamas. Diam. 45 mm. R. T. .1. Coll, 708. X 3. On the periiitome many 

 rows of ambulacra! and in addition interradial non-ambulacra! plates (.(impure with young. I'lulr 2, fig. 6). 



47. Archaeocidaris worlhnri Hall. Lower Carboniferous. From I'late 9, fig. 6. Partially restored. On the peri- 

 stome many rows of ambulacra! and in addition interradial non-ambulacra! plates. 



48. Melonechinus multijmrus (Norwood and Owen). Ixiwcr Carboniferous. Restored. From I'lai.- .V>, fig*. 7, g. 

 On the peristome many rows of ambulaeral and in addition two rows of inlerrtulial non-ambulacriil pliiti*; aml.iilarrnU 

 pass from two plates orally to many on the periphery of peristome in each area. 



49. Slrongylocentrotus drobachiensis (O. F. Miiller). Young. Diam. 1.2 mm. From Plate 3, fig. 1 1 . On the prri- 

 stome one row of ambulaeral plates. 



50. The same. York Harbor, Maine. Adult. Diam. 40 mm. R. T. J. Coll., 709. X 3. On th.- IHTIM.,,,,,. one 

 row of ambulacra! and scattered, small, non-ambulacral plates. 



51. Dermatodiadema anlillarum A. Agassiz. West Indies, 955 fath. Diam. 9 mm. R. T. J. Coll. ,670. X 8. On 

 the peristome one row of large ambulacra! plates. 



52. Echinarachnius parma (Lamarck). Eastport, Maine. R. T. J. Coll., 747. X 2. On tin- |Nri.it<>rni- no plate*. 



53. The same. X 6. Peristome enlarged. 



54. Echinocardium flavescens (Miiller). Adapted from Lov6n, 1874, Plate 3, fig. 34. On the |>ri.-i<>mi- many non- 

 ambulacral plates only. 



In text-figures 40, 41, 43, 45, 49, 52, and 54 the primordial intcrambulacral 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 around 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 ambulacra! 

 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), Lepidothrs 

 (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 only ambulaeral plates on the peristome is seen in Recent 

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

 cidarids, as shown by Mr. Agassiz (1904, p. 30, Plate 11, fig. 1). In the Cidaridne. however, 

 the primordial interambulacral plates are not retained at the base of the corona, but have l>een 



