148 ROBERT TRACY JACKSON ON ECHINI. 



the periproct as typical developing and adult features, and as variants in many species of fossil 

 and living Echini, the characters may be summarized as follows. In the Ordovician Bothrio- 

 cidaris archaica the oculars form a continuous ring with genitals on their dorsal border (text- 

 fig. 162). In the Devonian to Carboniferous usualty, as in Melonechinus (text-fig. 163), oculars 

 and genitals all reach the periproct and the corona. In the Mesozoic usually, as in Cidaris 

 coronata (text-fig. 164), the genitals alone reach the periproct and oculars are all exsert. In the 

 Recent, oculars are all exsert in the young individual. In the adult typically all oculars may 

 still be exsert, or one, or more to all plates may travel in so as to reach the periproct, and the 

 incoming, barring relatively rare exceptions, is in a perfectly definite order. Different species 

 or genera attain as a character definite points along this line of dilTerential development, and 

 selected cases of coming in are shown in text-figs. 165 to 169. All plates may be typicallj' 

 exsert, as in Echinus escidentus (text-fig. 116, p. 117). One plate may be typically insert, 

 when it is either ocular I, as in Echinus niagellanicus (text-fig. 165), or ocular V, as in Echino- 



Text-figs. 162-175. — Ocular plate arrangement in typical Echini. Each figure represents the typical character of 

 the several species represented. 



162. Bothriocidaris archaica sp. nov. Ordovician, Russia. From Plate 1, fig. 2. X 7. Oculars very large, meeting 

 in a ring, genitals small, dorsal to the oculars (p. 87). 



163. Melonechinus mulliporus (Norwood and Owen). Lower Carboniferous, St. Louis, Missouri. From Plate 50, 

 fig. 6. X 3. Oculars and genitals all meet the periproct and corona. Compare text-fig. 169. Genitals have many pores. 



164. Cidaris coronata Goldfuss. White Jura, Sonntheim. Stuttgart Museum 9,782. Enlarged. All oculars exsert. 

 Plates of periproct in place, very rare for fossils of the genus (pp. 96, 174). 



165. Echinus magellanicus ThiWppi. Patagonia. Diam. 26 mm. R. T. J. Coll., 773. X 2.7. Ocular I insert (p. 119). 



166. Slrongylocentrotuit franciscanu-s A. Agassiz. California. Diam. 112 mm. R. T. J. Coll., 699. X 1.7. Oculars 

 I, V insert (p. 144). 



167. Arbacia nigra (Molina). Chili. Diam. 92 mm. R. T. J. Coll., 796. X 2.6. Oculars I, V, IV insert (p. 116). 



168. Acrosalenia pseudodecorala Cotteau. Bathonien, France. (After Cotteau, 1S7.5-'S0, Plate 246, fig. 6.) Oculars 

 I, V, IV, II insert. Suranal plate is dorsal to genital 3 (p. 112). 



169. Phyllacanlhusbacidosa (Lamarck). Mauritius. Diam. 41 mm. R. T. J. Coll., 695. X 2.7. All oculars insert. 

 (Compare text-fig. 163, p. 102; Plate 3, figs. .3, 4.) 



170. Phortnosoma placenta Wyville Thomson. Off Cape Sable to Cape May, 956 fath. Diam. 56 mm. R. T. .1. 

 Coll., 707. X 3.5. All oculars insert and interspaces exist between oculars and genitals so that the jieriproct reaches the 

 interambulacra. Genitals are split ventrally. Ambidacral plates are simple primaries in the placogenous zone. Inter- 

 ambulacral plates originate again.st oculars as iisual (pp. 63, 110, 177; Plate 3, fig. 8). 



171. IIolectypHS hemisphericus Desor. Inferior Oolite, Leckhampton Hill, England. Diam. 28 mm. R. T. J. Coll., 

 738. X 9. Oculars all excluded from the center by genitals (p. 170). 



172. Cassidulus subquadratus Conrad. Upper Cretaceous, Holly Springs, Mississippi, .\fter W. B. Clark, 1893, Plate 

 31, fig. Ih. Enlarged. Oculars I, V reach the center, other oculars are excluded from the center by the genitals. 



173. Toxobrissus pacificus A. Agassiz. Adapted from A. Agassiz, 1904, text-fig. 279, p. 193. Enlarged. Oculars I, 

 V are virtually insert, although actually ocular I is shut out by the posterior extension of genital 2. 



174. Micrnslercorangiiineum {Lamarck) . Cretaceous, England. Length 55 mm. R. T. J. Coll., 521. X 6.7. Oc\i- 

 lars I, V, IV reach the center, others excluded from the center by the genitals. Compare text-fig. 167 (p. 92). 



175. Ananchyles ovalus (Leske). Cretaceous, Sussex, England. Length 67 mm. Student Laboratory Coll., 172, 

 Harvard University. X 2.4. Oculars I, V, IV, II reach the center, III only being excluded from the center by the genitals. 

 Compare text-figs. 126 and 168 (pp. 92, 167). 



