MANUAL OF ZOOLOGY. 



Upper Silurian, and Dithyrocaris in the Carboniferous Limestone. All 

 these forms, with other similar ones, are believed to be most closely allied 

 to the recent Apus and Nebalia. The genus Estheria, represented by 

 many forms from the Devonian period to the present day, is also to be 

 referred here. 



4. Trilobita. The Trilobites are exclusively Palaeozoic fossils. In the 

 Upper Cambrian rocks the so-called "primordial zone" there occurs a 

 singular group of Trilobites the so-called primordial Trilobites dis- 

 tinguished by the possession of many larval characters. In the Lower and 

 Upper Silurian rocks the Trilobites attain their maximum of develop- 

 ment. They are still well represented in the Devonian rocks ; but they 

 die out completely before the close of the Carboniferous epoch, being rep- 

 resented in the Mountain Limestone by four genera only (Phillipsia^ 

 Brachymetopus ; Proettts, and Griffit hides). 



5. Eurypterida. These, like the last, are entirely Palaeozoic, attaining 

 their maximum in the Upper Silurian and Devonian formations, and dying 

 out in the Carboniferous rocks. Pterygotus, Eurypterus, and Slimonia 

 are the most characteristic genera. 



6. Xiphosura. The genus Limulus commenced, as far as is yet known, 

 in the Permian period, and- has survived up to the present day. Its first 

 appearance, therefore, was just at the close of the Palaeozoic epoch. Of 

 the remaining genera, which constitute with Limulus this sub -order, 

 Belinurus, Eupro'dps, and Prestwichia, are Palaeozoic, and are not known 

 to occur out of the Carboniferous rocks. The genus Neolimttlus is 

 Upper Silurian. 



7. Amphipoda. The oldest known Amphipod is the Necrogammarus of 

 the Upper Silurian. 



8. Isopoda. The earliest known Isopod is the Praarcturus of the De- 

 vonian rocks. 



9. Stomapoda. This order is doubtfully represented in the Carbon- 

 iferous rocks by the genus Palceocaris, and by some allied types. 



10. Decapoda. The Macrurous Decapods commence their existence in 

 the Carboniferous period, or perhaps in the Devonian, with a few Prawn- 

 like forms ; and the Brachyura seem to have existed at the same period. 

 The Decapoda are, however, well represented, in all their three tribes, in 

 the Secondary and Tertiary epochs, attaining their maximum at the pres- 

 ent day. The London Clay (Eocene) is especially rich in the remains of 

 Macrura and Brachyura, 



CHAPTER XXXV. 



ARACHNID A. 



CLASS II. ARACHNIDA. The Arachnida including the Spi- 

 ders, Scorpions, Mites, &c. possess almost all the essential 

 characters of the Crustacea, to which they are very closely 

 allied. Thus, the body is divided into a variable number of 

 somites, some of which are always provided with articulated 

 appendages. A pair of ganglia is primitively developed in 

 each somite, and the neural system is placed ventrally. The 



