17-30. Development of Trinonira. Figures from Joscuim Barranpn, Systeme Silurien du Cen- 
tre de la Bohéme, I., Pl. VIL, XXX., Prague et Paris, 1852. 
17-22. Sao hirsuta. 
17. Youngest stage. The body is unsegmented, the future thorax being only faintly indicated by transverse 
furrows in the hinder part of the median lobe, and by three pairs of minute lateral spines. The small 
upper figure shows the natural size of this stage, the figure on the right a profile view. 
18. Later stage. The thorax is now clearly differentiated from the head, and consists of three ankylosed segments. 
19. Older form. The hinder division of the body now consists of five ankylosed segments, with no division into 
thorax and pygidium. 
20. Still later stage, signalized by the appearance of two free segments behind the head, which allow a demarka- 
tion to be now drawn between a thorax, composed of free segments, and a pygidium of ankylosed segments. 
As the trilobite develops, the thorax gains new segments at the expense of the temporary pygidium, until 
in the adult the thorax contains seventeen segments, the permanent pygidium being formed of two. 
21. Stage with four free or thoracic segments, and three or four ankylosed (pygidium). 
22. Stage with six free and three ankylosed segments. 
23-30. Trinucleus ornatus. 
23. Young stage, with head and pygidium, no thoracie segments. The pygidium shows traces of segmentation 
indicating two or three rings. 
24. Older stage, with one thoracic segment. As development proceeds, new thoracic segments are interposed 
between the one last formed and the pygidium, until in the adult the thorax consists of six segments. 
These new thoracic segments are probably formed from the anterior part of the pygidium at the successive 
moults. 
. Stage with two thoracic segments. 
. Stage with three thoracie segments. 
. Stage with four thoracic segments. 
. Stage with five thoracic segments. 
- Smallest known individual with the full number (six) of thoracic segments. Natural size. 
. Fully grown adult. 
co bo bo bo bs 
oO ON GS Ot 
