PLATAN ASTER ORDOVICUS. 171 



a nearly related form with infero-marginalia small or absent, which has taken to 

 mud-eating; its stomach has become enlarged, and the apical interradii have had 

 to grow outwards to accommodate it. 



There can be no doubt that the Platanasteridae branched off from the Asterozoan 

 stem at a very early date, and had reached a high stage of development in the 

 Ordovician. That the family is far advanced in its lineage history is shown by : 



(1) The large size of the species. The type is the largest English species of 

 its time, and only the American Ordovician species of Promopalseaster approach it 

 in size. 



(2) The petaloid nature of the arms. Ruedemann (97, p. 35) has shown that 

 this is a gerontic character in Devonaster. 



(3) The enlarged mouth-angle plates comparable with those of Promopalseaster 

 (compare Text-fig. 55, p. 96). 



On the other hand the proximal ambulacralia are in a primitive condition. 

 Usually among the Asterozoa the first ambulacralia are thickened and help to 

 form a continuous ring-shaped mouth-frame. Here they are not thickened, and 

 diverge very considerably from the radial line, forming a deep V in each radius. 

 Similar divergent proximal ambulacralia are found in many other primitive 

 Asterozoa, and Bather (89, p. 165, fig. 3) has figured divergent ambulacralia as a 

 character of Edrioaster. 



The diagnostic characters of the two genera are as follows : 



Platanaster. — Arms fused at the base. Stout infero-marginalia present. 

 Madreporite apical. 



Palasteriscus. — No infero-marginalia. Madreporite oral. 



Genus PLATANASTER, novum. 



Generic Character*. — As above. 



1. Platanaster ordovicus, n. sp. Plate XIV, figs. 1—3; Text-figs. 114-118, 



120, 121. 



Material. — One well-preserved impression with counterpart in the collection 

 of the Museum of Practical Geology, Jermyn Street, no. 8238 (mould of apical 

 surface) and no. 25347 (mould of oral surface). 



General Appearance (Text-fig. 114). — Text-fig. 114 is a reconstruction of one 

 arm and the neighbouring interradii. The form is so well preserved that the 

 figure is almost an exact drawing from a cast of the specimen. The broad 

 petaloid shape of the arm is brought about by three factors — the great breadth of 

 the adambulacralia, the spindle shape of the deep ambulacra] channel, the decrease 

 in size of both the ambulacralia and adambulacralia from about the middle lenerth 



