MESOPAL^ASTER (?) KETLEYI. 101 



(1) The first of these species is a form found in the Wenlock (Middle Silurian) 

 which I place provisionally in the genus McsopaJxaster. 



Mesopalaeaster (?) ketleyi, n. sp. Text-fig. 59. 



Material. — Only one specimen is known, in the Museum of the Birmingham 

 University (Ketley Coll., no. 220). 



Specific Characters. — Primary radialia and interradialia large but irregular in 

 form. Radialia broken down. Supero- and infero-marginalia alternating. Large 

 intermarginalia apparently present. 



The lineage to which this form belongs obviously branched off from the 

 primitive stock before the primary radialia and interradialia lost their large size, 

 primitive form, and primitive position, and before the proximal supero-marginalia 

 were thrust apicalwards. Unfortunately there are no connecting links between it 

 and any known Ordovician form. The oral surface is not known, but the form 

 shows general resemblances to both "Hudsonaster'' and "Mesopalaeaste)-'" stock. 



Apical Surface (Text-fig. 59). — The specimen can best be orientated by the 

 position of the madreporite, which is situated between arms i and i[ of the 

 Text-fig. 59. Immediately proximal to the madreporite is a large irregularly 

 shaped primary interradial bordered on each side by an irregularly shaped 

 primary radial. The primary radialia and interradialia can be followed as a 

 circlet of prominent plates round the disc at a point approximately median 

 between the centre and the margin. They are all irregular in appearance, and 

 can be recognised mainly by position. Here and there a few accessory plates are 

 interspersed in the circlet. 



Proximal to the circlet the disc is sunk and occupied by plates of very various 

 size and shape. There is one large plate which occupies the centre which may be 

 a centrale, but taken as a whole it is difficult to say that the plates in the sunk 

 area have any definite arrangement. The sunk area is about I'G mm. in diameter. 

 The arms vary considerably in appearance. 



The key to the structure exhibited by the arms appears to be given by the 

 interradius between arms v and iv. The ossicles on the extreme right side of 

 arm v are clearly infero-marginalia with the supero-marginalia immediately above 

 them. The outside ossicles of the left side of arm iv are also infero-marginalia. In 

 the axil between the arms is a large plate which at first sight suggests the appear- 

 ance of being a large odontophor which has been thrust somewhat upwards and 

 outwards. If this were so, the form would obviously be akin to one of the 

 Hudsonasteridge. I do not think, however, that this ossicle really belongs to the 

 oral surface. If it did, it would exhibit articulation, on its apical surface, for the 

 fitting of the covering supero-marginalia. It certainly does not do this, and 



