38 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Number and character of rays. In the geologically oldest and the 

 most primitive asterids as well there are always 5 rays, no multirayed 

 form being known back of the middle of Siluric time. In the most 

 primitive genus of Phanerozonia, Hudsonaster, the rays are short and 

 stout, there is a large disk without interbrachial arcs, and the skele- 

 ton is simple but ponderous. From this type originate those with 5 

 slender rays and also forms with more or less large interbrachial arcs, 

 the pentagonal starfishes. 



Long, slender, and flexible-rayed genera without marginalia are 

 present as early as the most primitive Phanerozonia, i.e., as early as 

 the older Middle Ordovicic, indicating that the two main types of 

 asterid development arose still earlier. The same is true for the 

 ophiurid ancestors (Auluroidea). In fact, slender-rayed Crypto- 

 zonia are more commonly seen in the Middle Ordovicic than are the 

 Phanerozonia. 



Multi-rayed Phanerozonia with primitive arm construction appear 

 in the Middle Siluric (Wenlock) in Lepidaster. This genus has rays 

 which are short and stout, reminding one in this respect strongly 

 of the primitive 5-rayed forms. An insufficiently described 10- 

 rayed form (Trichotaster plumiformis) is recorded from the Middle 

 Siluric (Wenlock) of England. In Helianihaster of the Lower Devonic 

 there are from 14 to 16 rays and here they are long and slender, 

 while in the Upper Devonic there is a form with only 1 1 rays. Lepid- 

 astereUa of the Upper Devonic has 24 arms. 



All of the multi-rayed Cryptozonia have slender rays and rather 

 large disks. None are older than the Lower Devonic, where Medusaster 

 has from 12 to 15 rays, Echinasterias 25, Palxosolaster 27 to 29 

 (an Upper Devonic species referred doubtfully to this genus has 24 

 or 25) , Echinostella 28 or 29, and Echinodiscaster 29. 



Curiously, no multi-rayed starfish is known hi the Paleozoic later 

 than the Devonic. Further, the arms may be either odd or even in 

 number in the same genus or even species. How the living multi- 

 rayed starfishes introduce their rays is explained under Palseosolas- 

 teridse (p. 207). 



Verrill (1914: 12-17) holds that the multi-rayed forms arose as 

 " ' sports/ which have persisted by heredity and natural selection 

 because they were advantageous." The advantage lay in "the 

 increased number of ambulacral sucker-feet. " Most of these forms 

 are now living in shallow water among the rocks exposed to the surf. 

 He states: " This is true of the seven species of Heliaster, with very 

 numerous rays; and of Pycnopodia, with 20 to 24 rays; and of the 

 various shallow-water and littoral species of Solaster and Crossaster, 

 which usually have 9 to 15 rays (rarely 8 or less). It is also true of 

 the numerous 6-rayed species of Asterias, Pisaster, and allied genera." 



