NATURAL CLASSIFICATION OF THE TRILOBITES 111 



manner the weak points of each, furnishing strong reasons 

 as to why they are unnatural and therefore untenable. The 

 underlying principles of these early attempts at a classifica- 

 tion are here briefly summarized: (1) The first classification 

 of trilobites was advanced by Brongniart, 11 in 1822, in which 

 all the forms previously known as Entomolithus paradoxus 

 were shown to belong to five distinct genera. (2) Dalman, 16 

 in 1826, made two groups based upon the presence or 

 absence of eyes. (3) Quenstedt, 30 in 1837, recognized the 

 number of thoracic segments and the structure of the eyes 

 as of the greatest importance. (4) Milne -Ed wards, 28 in 

 1840, considered the power of enrolment as of prime value. 

 (5) Goldfuss, 20 in 1843, made three groups, depending on 

 the presence or absence of eyes and their structure. (6) Bur- 

 meister, 12 in 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, 17 in 1839, 

 was founded on the shape of the pleura, the presence or 

 absence of eyes and their structure. (8) The later classifi- 

 cation of the same author, 18 published in 1844, was based on 

 whether the abdomen was composed of fused or free seg- 

 ments, and the minor divisions depended chiefly on the struc- 

 ture of the eyes and the facial suture. (9) Corda, 15 in 1847, 

 placed all trilobites in two groups, one having an entire 

 pygidial margin, and the other with the pygidium lobed or 

 denticulate. (10) McCoy, 25 in 1849, took the presence or 

 absence of a facet on the pleura for a divisional character. 

 As this is an indication of the power or the inability of 

 enrolment, it does not differ materially from the schemes of 

 Milne-Edwards and Burmeister. 



Zittel, 35 in a historical review brought down to 1885, in- 

 cludes in addition the schemes of Barrande and Salter, and 

 remarks that the basis of Barrande 's general grouping, 

 namely, the structure of the pleura, has neither a high phys- 

 iological nor morphological meaning. Both Barrande and 

 Salter recognize nearly the same families, with slight differ- 



