17(5 



THE PROGRESS OF DEGRADATIOJf. 



gradation, the four ichthyic limbs are first gathered irto a 

 cluster, and hung about the neck* 



Fig. 5S. 



TAIL OF PERCH. 



* In the following diagram a few simple lines serve to exhibit the pro- 

 gress of degradation. Fig. a represents the symmetrical placoids of the 

 Silurian period, consisting of head, neck, body, tail, fore limbs and 

 hinder limbs ; fig. b represents those heterocercal ganoids of the Old Red 

 Sandstone, Coal Measures, and Permian system, in which the neck is ex- 

 tinguished, and the fore limbs stuck on to the occiput ; fig. c, those ho- 

 mocercal ganoids of the Trias, Lias, Oolite, and Wealden, whose tails 

 spread out into broad terminal processes, without homologue in the higher 

 animals ; fig. d, those acanthopterygii of the Chalk that, in addition to 

 the non-homological processes, have both fore limbs and hinder limbs 

 stuck round the head ; while fig. e represents the asymmetrical platessa 

 of the same period, with one of its eyes in the middle of its head, and tht» 

 other thrust out to the side. 



[The first two figures may be now reasonably regarded as contempo- 

 rary. — Ed.] 



Fig. 59. 

 Silurian. Old Re^l, Arc. Lias, (fee. Cretaceous. 







B. 



o_ 



B: 



^^ 



^ 



I 



