456 Mr. W. P. Sladen on Astrophiura permira. [June 20, 



early differentiated into the two groups of Apneumatoccela and Pneu- 

 matocoela, the former being the root-stock of the modern Elasmo- 

 branchii, while the latter, by acquiring rudimentary and more or less 

 functional lungs, became the primitive double-breathers from which 

 have been derived the Ganoidei and the Amphibia. From the primi- 

 tive Ganoidei were derived the Teleosteoid Ganoids and eventually the 

 Teleostei also, their originally complex swim-bladders becoming gradu- 

 ally devoted to other functions, while the Selachoidei may be regarded 

 as the but little modified descendants of the original progenitors of the 

 order. The close correspondence that exists between Polyodon and the 

 Selachii is not incompatible with these views, but may be the result of 

 tbe persistence in both of structures originally possessed by their 

 primitive ancestor. Two facts in tbe cranial anatomy of Polyodon 

 are not easy to explain, viz., the formation of the upper jaw and the 

 existence of the " orbitar process." The union of the pterygoid pro- 

 cesses in a median symphysis may have been the primitive condition of 

 the jaws in the ancestral form (x), but that while persistent in 

 Polyodon and in the Selachii, it was superseded by a different arrange- 

 ment, viz., the union of the pterygoid processes with retral palatine 

 outgrowths in most Ganoidei, and in all Teleostei and Amphibia. 

 Neither is it easy to account for the retention of the " orbitar process." 

 It may have been an adaptive modification correlated with a suctorial 

 mouth in the larval or adult forms of those Ganoids that were first 

 differentiated from the Amphibian stem, and independently developed ; 

 or it may have been possessed by, and similarly functioned in, the 

 primitive Pneumatoccela, but has become obsolete in all their descen- 

 dants, except Polyodon and the Anura. Thus it would appear that the 

 Polyodontidaa constitute a remarkably central group. They retain not 

 a few of the characters which we may assume to have belonged to the 

 primitive stock out of which were evolved the three most important 

 groups of Ichthyopsida, combined, however, with a certain amount of 

 specialization ; nor are they altogether without indications of retro- 

 gression. 



V. " On Astrophiura permira, an Echinoderm-form intermediate 

 between Ophiuroidea and Asteroidea." By W. Percy 

 Sladen, F.L.S., F.G.S. Communicated by Professor 

 Duncan, F.R.S. Received June 18, 1878. 



(Abstract.) 



The following peculiarities of structure presented by the anatomy 

 of the echinoderm above described are noteworthy : — 



1, The combination of ophiuroid disk- and arm-structure within a 

 pentagonal asteroid form of body. 



