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BULLETIN OF THE 
Order Trilobita. Mouth furnished with a large hypostoma and 
four pairs (as far as known) of appendages. Thoracic segments, 
2 — 26, bearing jointed legs with attached branchiæ. Abdomen formed 
of anchylosed segments, 2 (?) — 28. hearing articulated appendages. 
The following tabulation «ives the characters presented by each 
order, 
XIPHOSURA. Ex. Limulus (fossil and living). 
. Eyes sessile, compound. 
. Ocelli distinctly seen. 
. All the limbs serving as mouth organs. 
+ All the thoracic segments bearing branchiæ or reproductive organs, 
. Other segments devoid of any appendages, 
. Thoracic segments anchylosed. 
. Abdominal segments unanchylosed and rudimentary. 
. Metastoma rudimentary. 
Eurvrreripa. Ex. Pterogotus, Hurypterus (fossil, extinct). 
. Eyes sess 
. Ocelli distinctly seen. 
. All the limbs serving as mouth organs. 
. Anterior thoracic segments bearing branchiæ or reproductive organs. 
ile, compound. 
Other segments devoid of any appendages. 
. Thoracic segments unanchylosed. 
+ Abdominal segments free and well developed. 
. Metastoma large. 
TRILOBITA. Ex. Asaphus, etc. (fossil, extinct). 
. Eyes sessile, compound. 
. Ocelli, unknown. 
. Cephalic limbs serving as mouth organs. 
. Thoracic segments bearing jointed legs and attached branchiæ, 
» All segments provided with appendages. 
. Thoracic segments unanchylosed. 
+ Abdominal segments anchylosed and bearing jointed appendages. 
. Hypostoma large. (Metastoma unknown.) 
The agreement in the structure of the cephalic appendages is 
taken as the basis of the union of the three groups under one head. 
The differences between the Zrilobita and the two remaining groups 
are very marked, especially in the thoracico-abdominal regions. "These, 
united with the great development of the Zrilobita as expressed 
in its large number of families and genera, are considered as sepa- 
rating it from a sub-group formed of Xiphosura and Burypterida, 
This is essentially the arrangement made by Woodward and other 
zoólogists, and one to which we subscribe, except that we would go 
