34 ROBERT TRACY JACKSON ON ECHINI. 



Cidaris. Mr. A. Agassiz (1904) in the Panamic Echini shows the same system in young Poro- 

 cidaris cobosi A. Ag. (his Plate 12, fig. 2), young Saknocidaris miliaris A. Ag. (his Plate 16, 

 fig. 1), young Phormosoma placenta A. Ag. (my Plate 3, fig. 10). He also shows the interesting 

 fact that the same order of ambulacral plates exists around the mouth in the adults of Cidaris 

 (Dorocidaris) panamensis (A. Ag.) (his Plate 3, fig. 4), Porocidaris milleri A. Ag. (his Plate 7, 

 fig. 6), Echinosoma hispidum (A. Ag.) (his Plate 41), Phormosoma zeylandiae A. Ag. (his 

 Plate 50, fig. 1), and Kamptosoma indistinclum A. Ag. (his Plate 50, fig. 1). 



It is true that in the adult of most of the regular Echini, where the primordial ambulacral 

 plates surround the mouth, usually more or less isolated, they have not retained the relative 

 proportion of the Love"n system seen in the young, and from them one could not orient a sea- 

 urchin. This has been dwelt upon at some length because in my studies of Bothriocidaris 

 archaica (Plate 1, fig. 1) it was found that the ambulacral plates around the mouth are arranged 

 according to the Lov6n law, which shows that it already was at work in the oldest known sea- 

 urchin. It is worth mentioning that I was ignorant of the law at the time the drawing was made 

 so that no prejudice came in to influence the making of the drawing. 



Love'n maintained that the sea-urchin could also be oriented by the relative size and posi- 

 tion of the ten interambulacral plates found at the base of the corona in regular Echini, the 

 la, 2a, 36, 4a, 56 plates being smaller, the 16, 26, 3a, 46, 5a larger. This is often true, but often 

 not true, and it seems that the law has no general application. Loven's laws of orientation 

 were carefully studied to see if correct orientation could thus be obtained in the Palaeozoic 

 types, but with the exception of Bothriocidaris without success. In Palaeozoic genera, as far 

 as known, ambulacral plates always extend on to the actinostome as far as the mouth, as shown 

 in Bothriocidaris, Hyattechinus, Pholidechinus, Melonechinus, and Lepidesthes, and if suffi- 

 ciently well preserved, the orientation by primordial ambulacral plates might be applied, as 

 in Bothriocidaris. This, however, must await future study. 



In the cases of Lepidesthes formosa (Plate 68, fig. 5), L. colktti (Plate 71, fig. 1), Meek- 

 echinus elegans (Plate 76, figs. 1, 6), and Lovenechinus lacazei (text-fig. 240), the specimens are 

 correctly oriented by the presence of a madreporite. Bothriocidaris archaica (Plate 1, fig. 1) 

 is oriented by the relation of the primordial ambulacral plates; Bothriocidaris pahleni and 

 B. globulus (Plate 1, figs. 6, 9) by the large ocular plate which, as shown in B. archaica, overlies 

 ambulacrum III. Hyattechinus beecheri (Plate 24, figs. 5-8) is oriented by the plane of 

 bilateral symmetry through an ambulacrum and posterior interambulacrum. Echinocystites 

 pomum (Plate 20, fig. 1) is oriented by the eccentric anal area situated in an interambu- 

 lacrum, presumably the odd posterior. The other species of Palaeozoic Echini figured cannot 

 strictly be said to be oriented, as the letters designating areas are selected arbitrarily in each 

 specimen without regard to a definite axis, which is an unknown quantity. The letters are 

 used simply as a convenience in description for reference to specific areas. 



