TRILOBITA. 527 



tirely from those borne on the same region in any of the Mero- 

 stojnata. Again, the Trilobites have no " ocelli," and have a well- 

 developed hypostome. On the other hand, the thoracic somites 

 in the Limuloids may be completely anchylosed (as they are in 

 Limulus itself) ; ocelli are present as well as compound eyes ; and 

 the hypostome is rudimentary or absent. Upon the whole, in spite 

 of the above-mentioned differences, it would seem that the Trilo- 

 bites are more nearly related to the Merostomata than to any other 

 group of the Crustacea, and if, as maintained by Claus and other 

 high authorities, the latter are properly referable to the Arachnida, 

 it will be apparently necessary to transfer the Trilobita also to this 

 class of the Arthropoda. Apart, however, from the difficult ques- 

 tion as to whether the Limuloids and Eurypterids are referable to 

 the Arachnida or to the Crustacea, it may be questioned if there 

 is sufficient justification, in the present state of our knowledge, for 

 placing these two groups along with the Trilobites in the common 

 division to which Claus has given the name of Giga?itostraca. The 

 differences between the Trilobita on the one hand, and the Xipho- 

 sura and Emypterida on the other hand, are, at any rate, so 

 numerous that it would be a matter of difficulty to frame a defini- 

 tion of the Gigantostraca which would embrace these three orders, 

 and would at the same time exclude all the other groups of the 

 Crustacea. 



The general facts as to the distribution of the Trilobita in past 

 time are soon told. The Trilobites are exclusively Palaeozoic, their 

 range extending from the Upper Cambrian to the Permian. If the 

 problematical Palceopyge Ramsayi of the Longmynd beds be a Tri- 

 lobite, then the order has its commencement in the Lower Cambrian. 

 In the Upper Cambrian rocks the order attained a wonderful devel- 

 opment, the number of generic and specific types already known 

 from this formation being very large, while individuals are sometimes 

 extremely abundant. Some of these so-called " Primordial " Trilo- 

 bites attain a very great size, being only surpassed amongst later 

 forms by some species of Asaphus. They mostly belong to the 

 families of the Ag?iostidoe, Conocephalidce, and Paradoxidce, and to 

 the genera Taradoxides, Co?wcoryphe, Sao, Ellipsocephalus, Hydro- 

 cephalus, Arionellus, Dikellocephalus, &c. Along with these, however, 

 are genera such as Agnostus, which pass up into the Ordovician. 

 Some of the " Primordial " Trilobites are not so highly organised 

 as their successors, as shown by the occasional absence of eyes, 

 or want of the facial suture, by the imperfect segmentation of the 

 glabella, or by the multiplication or diminution of the number of 

 the body-rings ; but others do not exhibit any inferiority to those 

 of later periods. 



In the Ordovician and Silurian rocks the Trilobites attain their 



