164 



PANAMIC DEEP SKA KCIIINI. 



mentary embryonic meridosternum of Spatangoids, though the size of the 

 plates is reversed. In Abatus, a 2 is the larger as in I [emipneustes, in Echino- 

 spatagus, Dysaster, and Spatangoids generally, while in Pilematechinus, 6 2 



is the larger, as is the case in Offaster, Mieraster, and Hemiaster. 1 In 

 Holaster, Collyrites, Echinolampas, Cassidulus, the plates l>2 or a 2 are some- 

 times the larger. In Cardiaster Peroni there are five single plates, one 

 above another. In the plastron of Stegaster, 2 though a meridosternal 

 one, the labium, like that of Plexechinus, is separated from the sternum by 

 the posterior lateral ambulacra. In Cardiaster tenuiporus the plates alternate 

 below the labium. 



fa 







18 mm. 



Fio. 286. <'. Wvvillii. 



83 mm. 



Fig. 237. C. Loveni. 



It seems to me as if Lambert himself had given us tbe best possible proof 

 of the accuracy of Loven's view of the development of the amphisternal from 

 the meridosternal plastron. The development of the adult amphisternal 

 Abatus from a meridosternal young (PI. 99, figs. 1-5, 8) seems to settle this 

 question in favor of Loven's view. In the young Echinocardium (1.7 mm.) 

 figured by Loven' the plastron is already amphisternal. In the young 

 Abatus there is no difference in the interambulacra ; the primordial plate 

 is followed in each by two plates differing greatly in size. 



While in the odd interambulacrum of the recent Spatangoids the primor- 

 dial plate, the labium, is followed by the amphisternal plastron, in the older 

 Spatangoids, as in Collyrites, Ananchytes, and in the Nucleolidae or Clypeas- 

 troids there is no marked difference in the order and arrangement of the 

 interambulacral plates in any of the interambulacra ; though in the latter 

 the order of the interambulacral plates is often interrupted by the lateral 

 encroachment of the ambulacral plates. 



1 These data are taken from Lambert's interesting paper on the Plastron of Spatangoids. Bull. 

 Soc. des Sc. hist, et nat. de l'Yonne. 2 e semestre, 1892. 



2 See Lambert. 1. c. p. 6 * Pourtalesia. PI. XV, figs. 172, 173. 



