20 ROBERT TRACY JACKSON ON ECHINI. 



has characters approaching those of another, hybriditj' is often assumed as a cause. I have 

 found numerous cases of variation to the character of another species where hybridity cannot 

 be considered on account of geographical separation, and it is feU that we should be extremely 

 cautious in assuming hybridity when the same results might be attained by variation. From 

 my studies on Echini, all the evidence goes to show that variation is in perfectly definite lines, 

 mostly arrested or progressive in character. When variatTon is aberrant, it still follows defi- 

 nite lines of aberration, and sporadic variation is very rare. This opinion is based largeh' 

 on a detailed study of genital and ocular plates which were carefully examined in over 50,000 

 specimens of Echini (text-fig. 176, p. 153; p. 164). 



To the late Professor Sven Loven all students of the Echini owe a great debt for the keen 

 ej^esight and insight with which he studied these fascinating animals. His method was largely 

 a critical detailed study of plate structure and relations, and this method I have attempted to 

 follow in the study of Palaeozoic forms. Frequently sutures are so difficult to see and speci- 

 mens are so contorted or badly preserved in these ancient forms, that I can hardly hope to have 

 escaped mistakes; but this method has been followed out consistently and with every effort 

 faithfullj' to describe and figure the specimens. 



Loven worked with Recent or post-Palaeozoic Echini, in which the axes are known from 

 the presence of the madreporite as well as in many t^ypes by the bilateral symmetry. With 

 •known axes he devised a nomenclature of areas which is of very great value and convenience 

 in brevity and clearness of description. He numbered the ambulacral areas from I to V, Roman, 

 and the interambulacra from 1 to 5, Arabic. The enumeration passed from left to right, re- 

 volving like the hands of a watch, with the specimen viewed from below and the odd anterior 

 ambulacrum being III. This nomenclature is considered more fully under Morphology. 

 It is readily seen in its application to Goniocidaris (Plate 2, figs. 1-3). In dealing with Palaeo- 

 zoic types, there was usually no means of ascertaining the axes, for the madreporite was seen 

 in only four species. Since, in the absence of known axes, Loven's system could not be used, 

 a system of nomenclature like that given in mj^ earlier paper is followed. Taking any inter- 

 ambulacrum as A, the several areas are lettered from A to J inclusive, revolving like the hands 

 of a watch, with the specimen viewed from the dorsal side. When a specimen is viewed from 

 below, the letters naturally revolve in the reverse order. This is shown in its application to 

 Lovenechinus (Plate 39, figs. 4, 5). 



It may be well to call attention to the application of this nomenclature of areas to speci- 

 mens viewed from different points so that it may be clearly in mind. External views, or in- 

 ternal molds, viewed dorsally: the axes revolve clockwise, from left to right (Maccoya, Plate 

 34, fig. 4; Lovenechinus, Plate 39, fig. 5). External views, or internal molds, viewed ventrally: 

 the axes revolve anticlockwise, or from right to left (Melonechinus, Plate 55, fig. 1; Lovene- 

 chinus, Plate 39, fig. 4). In molds of the exterior, if dorsal: the axes revolve anticlockwise, from 



