INTRODU(rriON. 47 



the adauilmlaeral plates. Genera: Palieaster'ina, McCoy, Schanader, Meek and 

 Wortlieu, Sriiiichertia, Gregory. 



Family 'S. Aspiclosonialidae. 



Phanerozonia with alternate ambulacral ossicles, large marginal ossicles, and 

 extensive depressed interradial areas. Rays massive, petaloid, sub-petaloid, or tapering. 

 Genera: Aspidosoma, Goldhiss, PalteosteUa, Stiirtz, Tricliasteropsis, Eck. 



Family 4. Tseniasteridfe. 



Phanerozonia with alternate ambulacral ossicles. There is neither disc nor 

 interbrachial area. Large marginal plates, of whicli the inframarginals act as the 

 adambulacral ossicles. Rays long, petaloid or tapering gradually. Genera: Ttvniaster, 

 Billings, ? Sfenaster, Billings, Urasterella, McCoy, ProtaMeracantJitoii, Stiirtz, Salteraster, 

 Stiirtz. 



Order 2. CRYPTOZONIA. 



Family Lepidasteridse. 



Cryptozonia which are heavily plated, but in which there are no special marginal 

 plates. The ambulacral ossicles are alternate (or possibly sometimes opposite). The 

 disc is large and the rays short, thick, and blunt or clavate. There are no lateral 

 spines. The abactinal plates are granular and closely set. Genera : Lepidaster, 

 Forbes, Etheridgaster, Gregory. 



It does not seem profitable to discuss these classifications in any detail. Many 

 of the forms are not true Asteroidea, but merely possess a " starfish-shaped " body. 



The evidence afforded by a detailed study of ossicles of the ambulacral groove 

 and mouth-frame of forms such as Asphlosoma and Htenaster shows that certain 

 of these so-called Asteroidea are really transition-forms between the Asteroidea 

 and the Ophiuroidea, while other genera present characteristics which enable us 

 to place them at the base of the Astero-Ophiuroid stem. In fact, careful analysis 

 discovers that the true Asteroidea were represented in early Palaeozoic rimes by 

 but few genera and species. 



Mention has already been made (p. 18) of an important preliminary paper by 

 Jaekel. A more extended study of these important forms would be of great 

 interest. 



The Thiiid Period. 



In recent years Schondorf (60 — 68) and Sollas and Sollas (71, 72) have pub- 

 lished several valuable papers on various forms of Palaeozoic Asterozoa. The merit 

 of their work lies not only in its exact observation, but also in its suggestive 

 character. The task of all future observers in the group is much lightened by 

 these researches. 



Schondorf has re-investigated in particular (1) the various species oi Aspidosoma, 

 (2) the true Asteroidea of the Greywackes (Devonian) of the Rhine. He has 

 shown with respect to (2) that the true Asteroidea form a small compact group 



