42 BRITISH PALEOZOIC ASTEROZOA. 



Stiirtz placed all the Palaeozoic " Opliiuroidea " in one of the above sub-orders. 

 He, however, founded two new families of the Ophiur;^. 



The first of these contains forms which are Ophiuroid in appearance, but have 

 the halves of their vertebrae alternating with each other. Adopting a nomenclature 

 corresponding to that used by Bronn for the sub-division of the Asteroidea, 

 Stiirtz called this Family the Ophio-Encrinasteriae. 



The second Family he called the ProtophiurerB. This contains the great majority 

 of the remaining Palaeozoic Ophiuroidea. The primitive character of the forms is 

 shown by the imperfection of the union of the opposing halves of the vertebrte 

 and by the usual absence of dorsal, ventral, and mouth shields. Stiirtz regarded 

 the Protophiure^e as related to certain recent deep-sea genera — 0])hiotholi((, 

 Ophiogeron, Opliiohehi^, and Opldohyrsa — which at first sight have a similar structure 

 of their vertebrae, and do not possess dorsal shields. 



The Palseozoic Ophiuroid forms which have branched arms and a leathery skin^ 

 e.(/. Hellanthaster, \Yeve ?it first {79, 1893) placed among the recent Euryalte, which 

 possess characters similar in these respects. Later (80, 189'J) Stiirtz stated that 

 he was doubtful as to the exact position of these forms, which might belong either 

 to the Protophiurea? or to the Palae-Euryalidae. The observations made b}- Stiirtz 

 were limited by the nature of his material. The process of pyritisation frequently 

 destroyed detail in structure. The resemblances relied upon by Stiirtz to establish 

 relationships between living and fossil forms are only superficial, and when more 

 perfectly preserved specimens are obtained these assumed relationships can readily 

 be disproved. 



The classification of Stiirtz is given in detail below, together with that of 

 Gregory, Avho almost entirely adopted the views of Stiirtz on the Palaeozoic forms 

 which should be placed amongst the Ophiuroidea, l)ut took as his basis of 

 classification that proposed by Bell (10). Bell's classification divides Recent 

 Ophiuroidea according to the form of the articulating facets on the vertebrte. 

 There are three main Orders : 

 (1) Streptophiurae, in which the faces of the vertebra? have rudimentary knobs 

 and corresponding depressions, so that the arms can be 

 coiled in the vertical plane. These are regarded as the 

 most primitive of the Ophiuroidea. 



The Order includes the whole of the Ophiomyxida? and 

 a few genera of the Amphiuridge of Lyman. 



(2) Zygophiurae, in which the vertebral faces have knobs and pits which 



prevent their coiling in a vertical plane (Text- fig. 16, p. 18). 

 The Order includes the Ophioglyphida3 and the majority 

 of the Amphiuridae of Lyman. 



(3) Cladophiurse, in which the arms can be coiled as in (1), and are 



in most cases forked. No teeth ; the arm-spines are 



