554 CRUSTACEA. 



reproductive organs, and also served to conceal the leaf-like gills, 

 fragments of which are sometimes found in the fossil state. Accord- 

 ing to Fr. Schmidt, the segments immediately following the " oper- 

 culum " have their ventral side formed, each, by a pair of lamellar 

 plates, which meet in the middle line, and which are regarded by 

 this observer as modified thoracic appendages carrying branchiae. 

 According to the investigations of Mr Benjamin Peach, some 

 Eurypterids were further provided with well-developed pectinated 

 organs, apparently similar in form and structure to the so-called 

 " combs " (pectines) of the existing Scorpions. Lastly, the seven 

 hindmost segments of the body are undoubtedly devoid of appen- 

 dages of any kind, and the last of these is the " telson." The form 

 of this varies, being lanceolate or bilobate in Pterygotus and Slimonia, 

 but narrow and sword-shaped in Eurypterus and Stylonurus. 



There are at present no materials available for working out the 

 development of the Eurypterids, but there is every reason to sup- 

 pose that the berry-like bodies which are found in the Old Red 

 Sandstone of Scotland, and which have been described under the 

 name of Parka decipiens, are really the eggs of Crustaceans belong- 

 ing to this order. 



There can be no question as to the importance of the relation- 

 ships between the Eurypterids and the Scorpions ; and many high 

 authorities consider that the order Eurypterida should be removed 

 from the Crustacea and placed in the series of the Arachnida. 

 There is, however, no reason for doubting that the Eurypterids were 

 water-breathing animals, provided with branchiae ; and as the nature 

 of the respiratory organs constitutes, perhaps, the most weighty of 

 the distinctions between the closely related classes of the Crustacea 

 and the Arachnida, it would seem proper to regard the Eurypterida 

 as an ancient type of the Crustacea, in which are preserved certain 

 of the characters which must have been possessed by the ancestral 

 Arthropods from which these two classes originally diverged. 



The nature of the deposits in which the remains of Eurypterids 

 are found, and of the fossils associated with them, would prove that 

 these animals were essentially marine, their habits, probably, being 

 very similar to those of the existing King-crabs. It is, however, 

 possible that certain of the Eurypterids were inhabitants of brackish, 

 or even of purely fresh, waters. As regards their geological range, 

 the Eurypterids seem to have appeared for the first time in the 

 Ordovician rocks, but comparatively little is known of the early 

 representatives of the order. On the other hand, Eurypterids abound 

 in the Silurian rocks, and it is in this system of deposits that the 

 order seems to have attained its maximum development. Numer- 

 ous Devonian Eurypterids are also known, and the genus Eurypterus 

 is represented in the Carboniferous rocks, while a doubtful repre- 



