210 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 branchit^. Abdomen formed 

 of anchylosed segments, 2 (?) -28, bearing articulated appendages. 



The following tabulation gives the characters presented by each 

 order. 



XiPHOSUKA. Ex. Lhnulus (fossil and living). 



1. E}'es sessile, compound. 



2. Ocelli distinctly seen. 



3. All the limbs serving as mouth organs. 



4. All the thoracic segments bearing branchiae or reproductive organs. 



5. Other segments devoid of any appendages. 



6. Thoracic segments anchylosed. 



7. Abdominal segments unmichylosed and rudimentary. 



8. Metastoma rudimentary. 



EuRYPTERiDA. Ex. PteroQotus, Eurypterus (fossil, extinct). 



1. Eyes sessile, compound. 



2. Ocelli distinctly seen. 



3. All the limbs serving as mouth organs. 



4. Anterior thoracic segments bearing branchiae or reproductive organs. 



5. Other segments devoid of any appendages. 



6. Thoracic segments unanchylosed. 



7. Abdominal segments free and well developed. 



8. Metastoma large. 



Trilobita. Ex. Asaphus, etc. (fossil, extinct). 



1. Eyes sessile, compound. 



2. Ocelli, unknown. 



3. Cephalic limbs serving as mouth organs. 



4. Tlioracic segments bearing jointed legs and attached branchiae. 



5. All segments provided with appendages. 



6. Tlioracic segments unanchylosed. 



7. Abdominal segments anchylosed and bearing jointed appendages. 



8. Hypostoma large. (Metastoma unknown. ) 



The agreement in the structure of the cephalic appendages is 

 taken as the basis of the union of tbc three groups under one head. 

 The differences between the Trilobita and the two remaining groups 

 are very marked, especially in the thoracico-abdominal regions. These, 

 united with the great development of the Trilobita as expressed 

 in its large number of families and genera, are considered as sepa- 

 rating it from a sub-group formed of XipJiosura and Euryptterida. 

 This is essentially the arrangennmt made by Woodward and other 

 zoologists, and one to which we subscribe, except that we would go 



