538 Prof. H. H. Swinnerton — Classification of Trilobites. 



Whitaker, W. 1886. " Some Essex Well Sections" : Trans. Epping Eorest 

 and Essex Nat. Field Club, vol. iv, pp. 149-70. 



1889. The Geology of London and of part of the Thames Valley (Mem. 



Geol. Surv., 2 vols.). 



Wood, S. V. 1867. (1) A Memoir in explanation of the structure of the 

 Glacial and Post-Glacial beds, mapped in a Geological Survey of the 

 Ordnance Sheets Nos. 1 and 2 ; comprising the Thames Valley between 

 London and the sea, and the Valleys of the Lea, Boding, Eavensbourne, 

 Cray, Darent, Crouch, and Chelmer Bivers, and of the Blackwater Estuary 

 and other subordinate valleys ; incorporated with which is an Essay upon 

 the general structure of the Post-Glacial system over the East, South-East, 

 South, and part of the South-West of England. Folio ; 54 double pp. and 

 2 maps. January, 1867. MS. Memoir in Library of the Geological 

 Society. 



1870. " Observations on the Sequence of the Glacial Beds " (part ii) : 



Geol. Mag., Vol. VII, pp. 61-8, 1870. 



1881. " Further Bemarks on the Origin of the Valley System of the 



South-Eastern Half of England, prompted by the Besult of a Boring near 

 Witham in Essex" : Geol. Mag., n.s., Dec. II, Vol. VIII, pp. 502-5, 1881. 



1886. "On the Sand-pit at High Ongar, Essex, with a note on 



Mr. W. H. Dalton's Paper on the ' Blackwater Valley ' " : Trans. Epping 

 Forest and Essex Nat. Field Club, vol. iv, pp. 76-86. 



Woodward, H. B. 1903. " Geology " in the Victoria History of the County 

 of Essex, vol. i, pp. 1-24, map. 



1909. The Geology of the London District (being the area included in 



Sheets 1-4 of the Special Map of London) : Mem. Geol. Surv. England 

 and Wales, 1909, viii, 142 pp., 1 map. 



II. — Suggestions for a Revised Classification of Trilobites. 



By H. H. Swinnerton, D.Sc, F.G.S., F.Z.S., Professor of Geology, 

 University College, Nottingham. 



(Concluded from the November Number, p. 496.) 



SUB-ORDKR MESONACIDA. 



NEVADIA is the earliest and the most primitive member of the 

 Mesonacidse, and as shown by Walcott l may be regarded as 

 representing the ancestral type of this family. The same authority 

 shows that the Paradoxidse have descended from the Mesonacidse. 

 Redlichia 2 also exhibits features which link it with the Paradoxidse 

 and the Mesonacidse. 



Beecher 3 places Zacanthoides near Paradoxides. Woodward 4 suggests 

 that this genus descended from Holmia. Walcott 5 couples it with 

 Albertella as a member of the Paradoxidse which exhibits approxima- 

 tions to the Mesonacidse. Reed 6 agrees with this conception of its 

 affinities. Zacanthoides and to a less extent Albertella may therefore 

 be regarded as forms which, like Paradoxides, have descended from 

 Mesonacidse. They have, however, attained a more advanced stage 

 in caudalization, a feature which taken with their probable origin 

 from a different section of the Mesonacidse would justify the forma- 

 tion of a separate family for these two genera, viz. Zacanthoidse. 



1 Smiths. Misc. Coll., vol. liii, p. 249, 1910. 



2 Walcott, Proc. Wash. Acad. Sci., vol. vii, 1905; op. cit., 1910, p. 254. 



3 Op. cit., 1897, p. 191. 



4 Geol. Mag., 1902, p. 539. 



5 Op. cit., 1910, p. 254. 



6 Palceontologia Indica, ser. xv, vol. vii, p. 9, 1910. 



