52 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Oral armature always with the adambulacral pieces most con- 

 spicuous. 



This order is not known to appear before the Middle Ordovicic, 

 and in the Paleozoic contains the following families: 



Hudsonasteridae. Xenasteridae. 



Palaeasteridae. Neopalaeasteridae. 



Promopalaeasteridae. Palasterinidae. 



Mesopalaeasterinae. Lepidasteridae. 



Promopalaeasterinae. 

 Anorthasterinae. 



These seven families of Paleozoic asterids are all related to one 

 another and seemingly had their origin more or less directly in 

 Hudsonaster or a form very similar to it. Therefore the family 

 Hudsonasteridae is the primordial stock out of which all asterids 

 progressively developed. Hudsonaster is massively and simply 

 built, with the spines rudimentary and restricted to the adambula- 

 crals and inf ramarginals ; it is a small rigid asterid with few and 

 comparatively large ossicles, nearly all of which are of the pri- 

 mary skeleton. From it the evolutionary tendency was to get rid 

 of the rigidity and ponderosity through the comparative reduction 

 in size of the plates and the introduction of many smaller sec- 

 ondary pieces, along with an abundance of spines. With this ten- 

 dency once established, there arose several others: (1) elongation 

 of rays, with greater flexibility and greater power of locomotion 

 through the indefinite duplication of pairs of podia; (2) increase of 

 body cavity in three different ways. This took place through the 

 development of interbrachial areas (a) by the inward crowding of 

 the single interbrachial axillaries, followed by more and more pairs 

 of proximal inf ramarginals; (&) through the insertion of an indefinite 

 number of accessory ossicles between the adambulacrals and the 

 inf ramarginals, forcing the latter more and more outward; and 

 (c) through the introduction of supernumerary rays. The latter 

 development is probably abnormal, in that the tendency arose in 

 the later larval life as is explained elsewhere. (See p. 207.) With 

 these changes of the actinal side naturally must arise compensating 

 growths of the abactinal surface and accordingly here is seen the 

 introduction of accessory ossicles in various places. 



Starting with Hudsonaster, there is one phyletic line developed 

 through the elongation of rays, flexibility, and the increase in size 

 of the oral region by the (a) method from the primitive Hudson- 

 asteridae into the derived Palaeasteridae, Promopalaeasteridae, Xen- 

 asteridae, and Neopalaeasteridae. This is the most satisfactory line 

 of evolution, resulting in the greatest variety of genera. Again 

 with Hudsonaster as the radicle, another phyletic line is established 

 through the increase of the body cavity by the (6) method, giving 



