708 ARTHROPODA phylum vii 



Crustaceans, but any attempt to include them in this way under higher groups, 

 such as the Entomostraca, Malacostraca or Merostomata, results in such broad 

 generalities and looseness of definition as to render these divisions of little 

 value. The present state of knowledge of Trilobite structure and develop- 

 ment is in favour of assigning them nothing short of the rank of a subclass. 



In nearly every particular, the Trilobite is very primitive, and closely 

 agrees with a theoretical Crustacean ancestor. Its affinities are with the 

 known subclasses of the group, especially their lower orders, but its position 

 is not intermediate. The more primitive characters may be summarised as 

 follows : — (1) They are all free marine animals; (2) they have a definite con- 

 figuration ; (3) the larva is a protonauplius-like form ; (4) the body and 

 abdomen are richly segmented, and the number of segments is variable ; (5) 

 the head is typically pentamerous ; (6) the thorax and abdomen are always 

 distinct, the number of segments in each being variable ; (7) all segments 

 except the anal bear paired appendages ; (8) all appendages except antennules 

 are biramous ; and (10) the coxal elements of all limbs form gnathobases, 

 which become organs of manducation on the head. Walcott has recently 

 discovered (1912) the appendages of the Cambrian trilobitic genus Neolenus 

 and those of other Crustaceans, which indicate that the Trilobite is a 

 Crustacean intermediate in structural organisation between the Branchiopoda 

 and the Merostomata. 



Classification. — Barrande gives a complete r6sum6 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 Entomolithus paradoxus were shown to belong to five 

 distinct genera. (2) Dalman, 1826, made two groups, based upon 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 importance. (4) 

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

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

 absence of eyes and their structure. (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 diff"er matei'ially 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, 



