ECHINOIDEA. II. 85 



In tlie last iiained paper by Lambert he evidently does not lay so nmch stress on tliese two 

 different types of plastron since he piaces the typical nieridosternous Meinitliiastrr in his family 

 Acropidæ which otherwise comprises forms \vith the plastron plns on moins developpé, et dans le 

 premier cas toujonrs amphisternev; he considers the genns Mciiulliiastcr as ' nne forme profondément 

 niodifiée, avec tendance au retonr vers nn groupement homogene des assules interambulacraires et 

 dont la disposition exceptionellement niéridosterne n'a qn'une importance relative, incapable de pré- 

 valoir contre l'ensemble des autres caractéres, notamment le groupement des plaques apicales (p. 323). 

 This leads ns to consider more closely the systematic valne of the characters afforded by the apical 

 system in the Aleridosicnii. I ma\' then recall the differences occurring among the Ponrfalesiidir with 

 regard to the aj^ical sxstem: disconnected in the /-"c^^z-A/A-jw-species; compact in Echiiiocrcpis cu/irahr, 

 disconnected in Ec/i. sc/igcra ; compact in Sfcniopafagiis, disconnected in Spatagocystis. Even if it is 

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

 genns (for which reason I have made Ecliiiiocr. sctigrra the t\pe of a new genns, see above p. 84), 

 nobod\- will donbt 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 occnr rather great differences in 

 the strnctnre of the apical system — see e. g. the two fignres of apical systems of Urcc/iiniis iiarr- 

 sianiis given by Agassiz (Pan. Deep-Sea Ech. p. 156. Figs. 226 — 27). There can thus be no donbt that 

 the apical s\stem is of comparatively little systematic importance among the Ålcridostcrni^ and it 

 seems to nie very irrational to place the nieridosternous Mcmitliiastcr among the amphisternous 

 «.Arropidæ:- on account mainly of its apical system, the more so as it differs, indeed, only very little 

 from the normal strnctnre thereof in the Aiiaiichytidæ. Ivikewise the fascioles are of comparatively 

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

 Slcreoptieustes, the marginal fasciole of Calynuic. and the faet that in Ureelt, iiarcsiai/us some speci- 

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



It seems then beyond donbt that the nieridosternous and the amphisternous strnctnre of the 

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

 accordingly, we get in i\\e grow^oiiho^ Åleridostcrni: '^\ft ^{tuiiuliytJildæ (or Echii/ocoryf/iidee), Urccfihiidæ 

 and Ponrtalesiidæ, in the group of the Amphistcrni: the rest of the Spataitgidæ. (I cannot here enter 

 on a discnssion of the families of the Amphistcrtii). It is at once seen that these two main gronps 

 are very natnral, another sign of the correctness of using the strnctnre of the sternnm as the principal 

 character. 



Without giving detailed diagnoses of the families of Mcridostmii I nia)- point out what to me 

 appear their main characters. In the UncJiiiiidu- the second plate of all the interanibulacra is a single 

 plate — probably not the result of the fusion of the piates a. 2 and b. 2, as thonght by Loven, 

 but of a nieridosternous- arrangement of these piates in all the interanibulacra, as thonght by Lam- 

 bert'. The Urcchiitidcr thus represent a separate brancli from the Ainnichytidæ, characterized by the 



' Agassiz, itis true, doubts that Slefiwpa/agns is realh- a l'ourtalesiid, but — iu iiiy opinion without sufficient 

 reason. Gregory (in Ray Lankester's Treatise on Zoology. III. p. 321) piaces Echiiiocyepis anA Spa/agocystis in the family 

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

 absurd, that it needs no refutation. 



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

 réahté, je ne crois pas que le systéme périsoniatiqne interradial des Echiuides comporte uue seule plaque double, pas méine 



