ON THE OEIGIN OF VEETEBEATES FEOM AEAOHNIDS. 359 



be guided in the main by comparison with those related forms 

 whose internal anatomy is better known. 



If the metastomum of Pterygotus is homologous with the 

 similarly-named organs of scorpions, it must be derived from 

 the first two abdominal segments. If, as its position indicates, 

 it arises in front of the swimming legs, the latter would belong 

 to the third abdominal segment, and would, therefore, be homo- 

 logous with the pectines of Scorpio. If this be so, then the 

 four pairs of appendages in front of the oars would correspond 

 in number and uniformity of structure with the four pairs of 

 walking legs of Scorpio; consequently the great chelate 

 appendages would be homologous, as indicated by their struc- 

 ture and function with the chelse of Scorpions. The small 

 size of the four appendages of Pterygotus and the large size of 

 the anterior ones is not difficult to understand, because if a 

 Scorpion-like animal should gradually adopt the habit of 

 swimming on its back by means of its comb or any oar-like 

 appendage, the walking appendages would naturally decrease 

 in size, while the grasping ones would not be so readily affected 

 by such a change. There should be in Pterygotus, according 

 to the above view, a pair of chelicerse in front of the chelae ; 

 but, as in all Arachnids these appendages are very small, it is 

 not strange there is no trace of them in the fossil forms under 

 consideration. 



XIII. Arachnid Features of Pterichthys. — We have 

 been impressed with the way certain fossil fishes, such as 

 Pterichthys and allied forms, resemble the Merostomata,^ 



' That this external resemblance is real may be shown by the fact that 

 those most familiar with the subject, and the ones best able to judge, were 

 also greatly impressed by the same fact. 



The " genialer " Hugh Miller, the discoverer of Pterichthys, says (' Old 

 Red Sandstone,' p. 50), in comparing a Tribolite with Cephalaspis, "The 



fish and the Crustaceans are wonderfully alike They exhibit the 



points at which the plated fish is linked to the shelled Crustacean." 



Also Sir Roderick Murchison, when first shown specimens of Pterichthys, 

 wrote regarding them that, " if not fishes, they more clearly approach to Crusta- 

 ceans than to any other class." Again, " They [Cephalaspis and Pterichthys] 



