1 7 - 30. Development of Trilobita. Figures frutii Joachim Barbande, Systeme Silurien da Cen- 

 tre de la Bohane, I., PI. VII., XXX., Prague et Paris, 18.52. 



17-22. Sao hirsuta. 



17. YouHgest 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 tliree 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 dilferentiatcd from the head, and consists of three anhylosed segments. 



19. Older form. The hinder division of the body now consists of five ankylosed segments, with no division into 



thora.x 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 tempor.ary iiygidium, until 

 in the adult the thora.x contains seventeen segments, the permanent pygidium being formed of two. 



21. Stage with four free or thoracic segments, and three or four ankylosed (jiygidium). 



22. Stage with six free and three ankylosed segments. 

 23 - .30. Trinuchus ornatus. 



23. Young stage, with head and pygidium, no thoracic 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 tliora.x consists of six segments. 

 These new thoracic segments are probably formed from the anterior part of the pygidium at the successive 

 moults. 



25. Stage with two thoracic segments. 

 2(>. Stage with three thoracic segments. 

 27. Stage with four thoracic segments. 

 2S. Stage with five thoracic segments. 



29. Smallest known individual with the full number (six) of thoracic segments. Natural size. 



30. Fully grown adult. 



