OPHIOLEPIS. 



163 



almost unaccountable unfossiliferous character of the Trias in our area, and for the extinc- 

 tion of the last traces of the palaeozoic fauna."^ 



In connection with the subject of the British fossil forms of the Opliiuridse described 

 in this Monograph, attention may be drawn to the peculiarities of structure presented by 

 the anatomy of the living Asfrophiura permira. Mr. W. Percy Sladen/ P.L.S., states 

 that with regard to the latter. 



1. The combination of Ophiuroid disk- and arm-structure within a pentagonal Asteroid 

 form of body. 



2. The character of the ambulacra! system is Asteroid : the divisional plates not 

 only being homologous with, but resembling in the manner of their disposition, the 

 ambulacral plates of Asteroidea ; at the same time furnishing a highly suggestive repre- 

 sentation of their phylogenetic development. 



3. The rudimentary structure of the mouth-armature is more Asteroid than Ophiuroid 

 in general facies. Absence of teeth, jaw-plates, and jaws. 



4. An extension of the peritoneal cavity to the extremity of the functional portion 

 of the rays, that is to say, to the margin of the pentagonal body. 



5. The condition and aborted character of that portion of the brachial series which 

 is prolonged beyond the body-disk is extremely rudimentary. 



6. The continuity of the tentacular pore-system is limited to the disk only. 



The above characters, as Mr. Sladen points out, are clearly sufficient to stamp the 

 peculiarity of this extraordinary Echinoderm ; and, whilst excluding it from any known 

 group of genera by their remarkable nature and by the aberrant departure they present 

 from all previous types, are such as would seem to necessitate the relegation of the form 

 to a family apart by itself. 



To speak definitely, he adds, as to the exact position of intermediacy which the organism 

 holds between the Asteroidea and Ophiuroidea would obviously be premature, without a 

 more detailed examination of the internal anatomy than the present specimen in its dry 

 condition will permit, as well as some knowledge of the life-history of the form. It may, 

 however, be safely affirmed, without overstepping the bounds of due caution, that Astro- 

 pJduria bridges further over, from the Ophiuroid side, the differences which have separated 

 the two orders, than any previously described Starfish or Brittle-star. 



1 ' Memoir of Edward Forbes, F.R.S.,' by Dr. Wilson and Prof. Geikie, p. 418. 



2 " On Astrophmra pei-mira, an Echinoderm-form intermediate between Ophiuroidea and Asteroidea," 

 by W. Percy Sladen, F.L.S., F.G.S. Eeceived June 18th, 1878. Communicated by Professor Duncan, 

 F.R.S. 'Proc. Roy. Soc.,' 20th June, 18/8. 



22 



