LARVAL STAGES OF TRILOBITES 



177 



advanced structure of the youngest stages observed, the ele- 

 mentary forms of this genus are as yet unknown, and possibly 

 the extreme tenuity of the test in the protaspis has prevented 

 their preservation. In the same way the young of Trinu- 

 deus (figure 78) show a separate cephalon and pygidium, and 

 the specimens are in a much more advanced stage of develop- 

 ment than the protaspis of Proetus, shown on Plate IV, 

 figure 5. An evidence of age is furnished, also, in the trans- 

 vei-se shape of the head, which, in typical elementary forms, 

 is longer than wide instead of wider than long. 



78 



79 



81 



83 



82 



FiGL'RE 76. — Agnostus nudus Beyrich. (After Barrande.) 



Figure 77. — Agnostus rex Barrande. (After Barrande.) 



Figure 78. — Trinucleus ornatus Sternberg. (After Barrande.) 



FiGORE 79. — LTydrocephalus saturnoides Barrande. (After Barrande.) 



Figure 80. — Hydrocephalus carens Barrande. (After Barrande.) 



Figure 81. — Olenellus (Mesonacis) asaphoides Emmons; Ford collection. 



(Original X 30.) 

 Figure 82. — Olenellus (Mesonacis) asaphoides Emmons. (After Ford.) 

 FiGURio 83. — Olenellus (Mesonacis) asaphoides Emmons. (After Walcott.) 



The youngest specimens of Arethusina KonincM, figured 

 by Barrande,^ are 2 mm. or upward in length, and have 

 seven or more free thoracic segments, with the cephalon 

 wider than long. The facts of ontogeny show that younger 

 stages must be admitted in which the number of segments 

 diminishes to nothing, continuing down to a form agreeing 

 with the protaspis of other genera. 



12 



