328 THE VOYAGE OF H.M.S. CHALLENGER. 



deep water. lu the Silurian, however, we have species whicli are figured with a double 

 row of alteruatiug, angular under arm plates, an arrangement found in no living species.-^ 

 Such are Ptilonaster princeps and Eugaster logani. It is plain that simple armed 

 Astrophytons begin as low as the coal ; for Onychastei- jlexilis, Meek and Worthen, 

 evidently belongs in this group. 



Two French authors - have endeavoured to discriminate the separate pieces of genera 

 found in the middle Lias marles. In the absence of a general knowledge of the finer 

 anatomy of the hard parts, their attempt is of the most elementary character, liut one 

 which nevertheless deserves great praise, for in everything there must be a beginning, 

 and it is always creditable. They found some marles largely composed of this debris, a 

 most important fact, showing that the Triassic Ojihiuridse lived in herds, as they often 

 do now. There is one mouth shield which with much probaljility they determine as be- 

 longing to Ophioglypha. The parts referred to Ophiothrix may rather, perhaps, 

 belong to some genus near Opjliiacantha. It is partly with a view to aid similar 

 researches that I have given several plates of the skeletons of Ophiurans. 



1 Twentieth Report Regents of University of New York on State Cabinet, 1867, pi. ix. figs. 8, 9. 



2 Terquem et Berthelin, Etude microscopique, &c., Mem. Soc. Geol. de France, 2euie Serie, vol. x. p. 99, 1875 

 pi. xviii. figs. 22-25. 



