BUB-CLAM i TIIII.nUITA IIYI'UI'AIMA 



are biramOUS ; and (10) the eo\al elements of all limli.s form -n.-uli- 

 which become orgiius of mambu atioii on tin- head. 



Clii.wfirttfinii. Ban-ami.- givefl a complete iv-nni'- nf tin- rla-.-ilir;itions 

 applied to Trilobites down to ls."n. ami -h<>\\> in a very satisfartory manner 

 the weak points of each, furnishing strong reasons as to whv thev arc un- 

 natural and therefore untenable. The underlying principles of tln'-t- e.ulv 

 attempts at a classification are here briefly >ummarised. (1) The first classifi- 

 cation of Trilobites was advanced by Brongniart in 182l', in which all the 

 forms previously known as AVo/W/7/nrs- ^/v/i../-//.s were shown to belong to five 

 distinct genera. (2) Dalman, 182G, made two groups, based ii])on the presence 

 or absence of eyes. (3) Quenstedt, 1837, recognised the number of thoracic 

 segments and the structure of the eyes as of the greatest important 

 Milne-Edwards, 1840, considered the power of enrolment as of prime value. 

 (5) Goldfuss, 1843, established three groups depending on the preset 

 absence of eyes arid their structurae. (6) Burmeister, 1843, accepted the two 

 divisions of Milne-Edwards, and laid stress on the nature of the pleura and the 

 size of the pygidium. (7) Emmrich's first scheme, 1839, was founded on the 

 shape of the pleura, the presence or absence of eyes and their structure. (8) 

 The later classification of the same author, published in 1844, depended on 

 whether the abdomen was composed of fused or free segments, and the minor 

 divisions were based chiefly on the structure of the eyes and the facial suture. 

 (9) Corda, 1847, placed all Trilobites in two groups, one having an entire 

 pygidial margin, and the other with the pygidium lobed or denticulate. (10) 

 M'Coy, 1849, took the presence or absence of a facet on the pleura for a 

 divisional character. As this is an indication of the relative power of enrol- 

 ment, it does not differ materially from the schemes of Milne-Edwards and 

 Burmeister. Zittel, in a review brought down to 1885, includes in addition 

 the schemes of (11) Barrande, 1850, and (12) Salter, 18G4, and remarks that 

 the basis of Barrande's general grouping, namely, the structure of the pleura, 

 has neither a high physiological nor a morphological significance. Both 

 Barrande and Salter recognise nearly the same families, with slight differences, 

 and the latter adopts a division into two lines, based on the number of body 

 rings and size of the pygidium. These include and are themselves included 

 in four groups, founded on the presence and form of the facial suture and the 

 structure of the eyes. (13) Chapman, in 1889, proposed four sub-on 1 

 primary groups based purely upon arbitrary features of general structure and 

 configuration, especially the form of the glabella, whether wide, conical, or 

 enlarged. (14) Haeckel, in 1896, divided the Trilobites into two orders 

 based upon the presence or absence of a functional pygidium. 



In the classification here adopted, the families as defined by Salter and 

 Barrande are in the main adhered to, and the number corresponds very closely 

 with that in Zittel's ILim/lnn-h </< r /'uliicnnfnlnifii', and also with that in the 

 Grundziige of the same author. The ordinal divisions, and the definition.- and 

 arrangements of families, are taken from the classification prepared by Beecher 

 in 1897, based upon the study of ontogeny and morphogenesis, as already 

 shown. 



Order 1. HYPOPARIA. Beecher. 



Free cheeks formimj a coiiH/tinm.-; iimniiinil ventral j>/te of the ccjihulon, am/ in 

 some forms also </.</< /*<////</ ,tvcr flu- ilnr*il side at the yenal //;//.<. >'/? vntral 



