130 STUDIES IN EVOLUTION 



so aberrant and offer such conspicuous differences from ordi- 

 nary trilobites that it was considered better to delay their 

 disposition until the variations in structure governing the 

 arrangement of families in the higher orders were clearly 

 shown. The degree of specialization of the glabella, of the 

 form and character of the fixed-cheeks, and the great range 

 in the number of segments in the thorax and pygidium are 

 strong evidence that we are dealing with the terminal genera 

 of the order, which must have attained its normal develop- 

 ment in pre-Cambrian times. Agnostus and Microdisous 

 have so many protaspidian and larval characters that they 

 must be considered more primitive than the other genera, 

 although in some respects they show a high degree of speciali- 

 zation and even degeneration, as will be noticed under the 

 family Agnostidse. Moreover, Harpes, in its elongate cepha- 

 lon, persistent ocelli, and many thoracic segments, is also 

 quite primitive. Trinucleus, with ocelli present only in 

 larval stages, a transverse cephalon, and genal spines belong- 

 ing to the free-cheeks, is considerably higher and properly 

 comes last in the order, thus making the arrangement of 

 families as follows: (1) Agnostidse (^Agnostus, Microdiscus, 

 Plate II, figures 9, 10); (2) Harpedidse (^Harpes, figure 11); 

 and (3) Trinucleidse (^Trinucleus, Ampyx, figures 12, 13). 



Diagnoses and Discussions. 



Sub-class Trilobita. 



Marine Crustacea, with a variable number of metameres; body 

 covered with a hard dorsal shell or crust, longitudinally trilo- 

 bate from the defined axis and pleura; cephalon, thorax, and abdo- 

 men distinct. Cephalon covered with acephalic shield composed 

 of a primitively pentamerous middle piece, the cranidium, and 

 two side pieces, or free-cheeks, which may be separate or united 

 in front, and carry the compound sessile eyes, when present; 

 cephalic appendages pediform, consisting of five pairs of limbs, 

 all biramous, and functioning as ambulatory and oral organs, 

 except the simple antennules, which are purely sensory. Upper 

 lip forming a well-developed hypostoma; under lip present. 



