SUB-CLASS I TRILOBLDA=TvViPoOr ARITA 623 
are biramous; and (10) the coxal elements of all limbs form gnathobases, 
which become organs of manducation on the head. 
Classification.—Barrande gives a complete résume of the classifications 
applied to Trilobites down to 1850, and shows in a very satisfactory manner 
the weak points of each, furnishing strong reasons as to why they are un- 
natural and therefore untenable. The underlying principles of these early 
attempts at a classification are here briefly summarised. (1) The first classifi- 
cation of Trilobites was advanced by Brongniart in 1822, in which all the 
forms previously known as Lntomolithus paradorus were shown to belong to five 
distinct genera. (2) Dalman, 1826, made two groups, based upon the presence 
or absence of eyes. (3) Quenstedt, 18537, recognised the number of thoracic 
segments and the structure of the eyes as of the greatest importance. (4) 
Milne-Edwards, 1840, considered the power of enrolment as of prime value. 
(5) Goldfuss, 1843, established three groups depending on the presence or 
absence of eyes and 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 cliefly 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, 1864, 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-orders or 
primary groups based purely upon arbitrary features of general structure and 
configuration, especially the form of the glabella, whether w ide, 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 Handbuch der Pulaeontologie, and also with that in the 
Grundziige of the same author. The ordinal divisions, and the definitions 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 forming a continuous marginal ventral plate of the cephalon, and in 
some forms also extending over the dorsal side at the genal angles. Suture ventral 
