SECTION C. 125 



possible, however, that the American Ordovician species named by Schuchert (85, 

 p. 146) Petraster? americanus, D'Orbigny, may be an earlier relation. 



This species is only known from a specimen which lies in the matrix in such a 

 manner as to show the inner apical aspect of the ventral plates. It has a com- 

 paratively large disc, stout month-angle plates with large tori and adambula- 

 cralia described as " articulated together by little processes and corresponding 

 sockets or sinuses." Neither drawing nor description gives sufficient detail to 

 make the exact relationships of the form certain, but it seems possible that if the 

 form were studied anew a more exact correspondence with Lepidaster would be 

 found. 



Section G. — Family Urasterellid^:, Schuchert ; 

 ,, Cnemidaotinidje, nova ; 

 „ Protarthrasteridj:, nova. 



In the Introductory Section to this Monograph I pointed out (e.g., p. 52) that 

 the Palceozoic Asterozoa include : 



(1) Forms which are more or less strictly comparable to the Asteroidea and 

 Ophiuroidea of the present day. 



(2) Forms which really represent branches of the Asterozoan stock not repre- 

 sented by modern survivors. 



The Families described in this section belong to an extinct branch stock at one 

 time with a world-wide distribution and of great importance. They must have 

 originated early because they form an important section of the earliest known 

 Asterozoa. The classified list of characters given below shows that they had 

 distinct Asteroid affinities masked frequently by the Ophiuroid (wriggling) mode 

 of life which they assumed. 



The following general characteristics of these Families may be enumerated : 



(a) Those which show the Relationships with the Asteroidea. 



(1) There is usually at least one row of marginalia. 



(2) The madreporite is apical in position. 



(3) The inter-adambulacral muscles have the same type of insertion 



as in many Recent Asteroidea. 



(4) The ambulacralia are always " asteroid." 



(b) Special Characters. 



(1) The disc is small and the arms are very long in proportion 



(see Text-fig. 89, p. 139). This gives the forms a very 

 " ophiuroid " appearance. 



(2) The adambulacralia are stout plates usually covering the whole 



of the oral surface of the arm and frequently also assisting 

 in forming the margin. Each almost always possesses a 



17 



