84 



ECHINOIDEA. II. 



essentialK from tliose of other Ponrtalesiæ '. — These differences in llie pedicellariæ are certainly not 

 verv iuiportant, and probably Ech. cuiirafa will also prove to have both ophicephalons and rostrate 

 pedicellariæ. The more important are the differences in the apical system and the bivial ambulacra, 

 so imjiortant, indeed, that it seems qnite nnnatnral to nnite the two species in one genns. I tliink 

 it necessary to create a new genns for scfigera. for which I may projjose the name Cystocrepis n. g. 

 Also the difference in the shape of the test is very conspicmnis, thongh perhaps not reliable for 

 a generic character. 



Regarding the systematic position of the family Poiirtalrsiid<r I qnite agree with de Meijere, 

 who has in a most skilful nianner discnssed the whole qnestion ( <Sibogas-Ecli. p. i6o — 71); it seems to 

 nie that he has shown be}ond donbt that the Poitrtalesiidæ represent a very special development 

 from the Aitaiichytida-, the highly interesting ^^wwa Stcniopatagiish^iwgm niany respects a transitional 

 form Ijetween the Poiirtalcsiidæ and the Aiiaiichyfidæ, thongh alread\- decidedly belonging to the 

 former famih'. (I can not agree with Agassiz, who thinks Stcrnopatagiis more related to the Anan- 

 chytidæ whereas, on the other hånd, he refers the genns Flrxcc/iimis — in my oj^inion midonbtedh- 

 an Urechinid ■ — to the Pourtalesiidæ). 



It is Lambert's merit to liave first emphasized (in his excellent <Étndes morphologiqnes snr 

 le plastron des Spatangides»)- that the difference between the meridosternons and the amphisternons 

 strnctnre of the plastron in the Spatangoids is of primary systematic importance, so that the whole of 

 the recent Spatangoids may be divided into Mcridosterni and Amp/iisfrn/i, names given by Loven, 

 who did not, however, cleai'ly point ont the importance of these different strnctnres, which he had 

 detected. The two types cannot be derived one from the other, bnt mnst have derived from forms 

 with a simple, nnmodified strnctnre of the odd interambnlacrnni, something like what is fonnd in 

 Dysastcr and the Cassidulidæ. To be snre, Agassiz (Pan. Deep-Sea Ech. p. 164) thinks that Ivambert 

 > has himself given ns the best jjossible proof of the accnracy of Lovén's \'iew of the development of 

 the amphisternal from the meridosternal plastron. The development of the adnlt amphisterual Abatns 

 from a meridosternal yonng (PI. 99. 1—5, 8) seems to settle this qnestion in favonr of Lovén's view». 

 Bnt, as is easily seen, the yonng Abatus represented in PI. 99. 3 does not show the slightest trace of 

 a meridosternons strnctnre, both the piates 5. a. 2 and b. 2 being in wide contact with the labrnni, 

 whereas the meridosternons strnctnre, as is well known, nieans that only one plate (b. 2) is in contact 

 with the onter end of the labrnm. The specimen fignred by Agassiz might perhaps be said to have 

 as yet no sternnm developed, the piates 5. a. 2 and b. 2 being rather small, thongh distinctly larger 

 than the following ones. At most this stage can show that the amphisternnm is derived from a primitive 

 strnctnre, where no sterniun is developed as yet; in this way Lamberts refers to the fignre of a 

 }onng Palæopnetistes cristatiis in the Blake -Echini (PI. XXI. 11) as .showing ^connnent on doit coni- 

 prendre le développement amphisterne dn plastron, qni procéde d'nn etat originaire on les plaqnes 

 ,sont semblables dans tontes les aires interradiales, comme chez les Cassidnlides . 



■ W'hether it is the ophicephalous pedicellariæ, which are brilliant glassy heails stamliiig out like miniature spheres 

 on the dark tests (Pan. Deep-Sea Ech. p. 147) I dåre not saj-. 



2 BuU. Soc. de l'Yonne. 1892. 



3 Note sur quelques Échinides crétacés du Madagascar. BuU. Soc. Géol. de France, 3. Ser. 24. 1896. p. 323. 



