E( HINOIDEA. II. 



85 



In the last named paper by Lambert he evidently does not lay so much stress on these two 

 different types of plastron since he places the typical meridosternous Menuthiaster in his family 

 Aeropidee which otherwise comprises forms with the plastron plus 011 moins developpe\ et dans le 

 premier cas toujours amphisterne : he considers the genus Menuthiaster as une forme profondement 

 modifiee, avec tendance an retonr vers nn groupement homogene des assules interambulacraires et 

 dont la disposition exceptionellement meridosterne n'a qu'une importance relative, incapable de pre- 

 valoir contre rensemble des autres caracteres, notamment le groupement des plaques apieales (p. 323). 

 This leads us to consider more closely the systematic value of the characters afforded by the apical 

 system in the Meridosterni. I may then recall the differences occurring among the Pourtalesiidce with 

 regard to the apical system: disconnected in the Pourtalesia-spedes; compact in Echinocrepis cuneata, 

 disconnected in Ecli. setigera; compact in Sternopatagus, disconnected in Spatagocystis. Even if it is 

 scarcely correct to admit species with compact and with disconnected apical systems into the same 

 genus (for which reason I have made Echinocr. setigera the type of a new genus, see above p. 84), 

 nobody will doubt that all these genera are very nearly related, and are rightly referred to the same 

 family '. — Even among specimens of the same species there may occur rather great differences in 

 the structure of the apical system - see e. g. the two figures of apical systems of Urechinus nare- 

 sianus given by Agassiz (Pan. Deep-Sea Ech. p. 156. Figs. 226 — 27). There can thus be 110 doubt that 

 the apical system is of comparatively little systematic importance among the Meridosterni, and it 

 seems to me very irrational to place the meridosternous Menuthiaster among the amphisternous 

 Aeropidce on account mainly of its apical system, the more so as it differs, indeed, only very little 

 from the normal structure thereof in the Ananchytidm. Likewise the fascioles are of comparatively 

 small systematic importance among the Meridosterni I may recall e. g. the subanal fasciole of 

 Stereopneustes, the marginal fasciole of Calymne, and the fact that in Urech. naresianus some speci- 

 mens have a subanal fasciole, while other specimens show no trace thereof. 



It seems then beyond doubt that the meridosternous and the amphisternous structure of the 

 plastron is the primary systematic character among the higher Spatangoids. On grouping the genera 

 accordingly, we get in the group of the Meridosterni: the Ananchythidce (or Echinocorythidee), Urechinidtz 

 and Pourtalesiidce, in the group of the Amphisterni: the rest of the Spatangidcr. (I cannot here enter 

 on a discussion of the families of the Amphisterni). It is at once seen that these two main groups 

 are verv natural, another sign of the correctness of using the structure of the sternum as the principal 

 character. 



Without giving detailed diagnoses of the families of Meridosterni I may point out what to me 

 appear their main characters. In the Urechinidce the second plate of all the interambulacra is a single 

 plate — probably not the result of the fusion of the plates a. 2 and b. 2, as thought by Loveu, 

 but of a meridosternous arrangement of these plates in all the interambulacra, as thought by Lam- 

 bert 2 . The Urechinidm thus represent a separate branch from the Ananchytidce, characterized by the 



1 Agassiz, it is true, doubts that Sternopatagus is really a Pourtalesiid, but - in my opinion without sufficient 

 reason. Gregory lin Ray Lankester's Treatise on Zoology. III. p. 321) places Echinocrepis and Spatagocystis in the family 

 Spatangidiv, even in two different sections, whereas Poui-talesia is kept as a distinct family. This classification is, indeed, so 

 absurd, that it needs no refutation. 



; In the great Monograph of Echinocorys I.Mem. Mus. d'hist. nat. de Belgique. II. 1903) p. 26 Lambert says: en 

 realite, je ne crois pas que le systeme perisomatique interradial des Echinides comporte une seule plaque double, pas meme 



