ECHINOIDEA. II. 85 



111 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 Menuthiastcr in his family 

 Acropidie which otherwise comprises forms with the plastron plus ou moins developpe, et dans le 

 premier cas tortjours amphisterne ; he considers the genus Menuthiaster as une forme profondement 

 modifiee, avec tendance an retour vers un groupement homogene des assules interambulacraires et 

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

 valoir centre I'ensemble des autres caracteres, notaniment le groupement des plaques apicales (p. 323). 

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

 system in the Mcridostcrni. I may then recall the differences occurring among the Pourtalcsiida with 

 regard to the apical system: disconnected in the / } our/alcsia-species; compact in Echinocrepis cuncata, 

 disconnected in Ech. sctigcra; compact in Stcrnopatagus, 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 no doubt that 

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

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

 o.Ai ; ropidcc .-> on account mainly of its apical system, the more so as it differs, indeed, only very little 

 from the normal structure thereof in the Ananchytida'. Likewise the fascicles are of comparatively 

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

 Stercopneiistcs, the marginal fascicle of Calymnc, and the fact that in Urech. naresianus some speci- 

 mens have a subanal fascicle, 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 Meridostcrni: the Ananchythida; (or Echinocorythidce), Urcchinid(r 

 and Pourtalcsiidcr, in the group of the Amphistcrni: the rest of the Spatangida;. (I cannot here enter 

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

 are very 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 Mcridostcrni I may point out what to me 

 appear their main characters. In the Urechinidcc 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 cmeridosternous arrangement of these plates in all the interambulacra, as thought by Lam- 

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



1 Agassi/,, it is true, doubts that Slernopatagus is really a Pourtalesiid, but in my opinion without sufficient 

 reason. Gregory (in Rav Lankester's Treatise on Zoology. III. p. 321) places Echinocrepis and Spatagocys/is in the family 

 Spatangidtz, even in two different sections, whereas Pourtalesia is kept as a distinct family. This classification is, indeed, so 

 absurd, that it needs no refutation. 



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

 realite, je ne crois pas que le systenie perisoniatique interradial des Echinides comporte uue seule plaque double, pas meuie 



