LIFE-FORMS OF THE PAST AND PRESENT. 
403 
[Figs. 1-9, copied and reduced from the plates illustrating Mr. Henry 
Woodward’s Monograph on the Merostomata, published in the volumes of 
the Palseontographical Society, 1867-72.] 
EXPLANATION OF PLATE XCI. 
[This plate is reproduced here by permission of the Council of the* 
Geological Society of London from a paper by Mr. H. Woodward, u On 
the Relationship of the Xiphosura to the Eurypterida, and to the Trilobita 
and Arachnida.” See “ Quart. Journ. Geol. Soc.,” 1872. Yol. XXVIII. 
p. 46.] 
Figs. 1-8. Trinucleus ornatus, Sternb. sp. (copied from Barrande’s 
“ Systeme Silurien du centre de la Boheme,” Prague, 1852, 4to, plate 30). 
Specimens arranged in series according to their supposed age. (All the 
stages figured byBarrande are not given here.) 
Fig. 1. Young individual, destitute of thoracic segments, composed of 
head-shield and pygidium only. 
„ 2. Another of the same stage, in which the genal or cheek-spines are 
developed. 
„ 3. Individual with one thoracic segment developed, but without the 
genal spines. 
„ 4. Another of the same stage, with the genal spines. 
„ 5. Individual with two thoracic segments, and in which the genal 
spines are present. 
„ 6. Individual with three thoracic segments, and possessing the genal 
spines. 
„ 7. Individual with five thoracic segments, but without genal spines. 
„ 8. Adult Trinucleus , with six thoracic segments and fully-developed 
genal spines. 
Figs. 9-15. iSao hirsuta , Barrande (copied from plate 7 of Barrande’s 
work above cited). Barrande figures twenty stages of this Trilobite, of 
which we have only reproduced seven. 
Fig. 9. First stage. A young individual in which the limit of the head- 
shield is not indicated as separating it from the pygidium. 
„ 10. Second stage. Young individual with the head-shield separated, 
and having inclinations of three soldered segments to the 
pygidium. 
„ 11. Third stage, in which the genal angles of the head and the spiny 
border of the pygidium are well seen, and four or five soldered 
segments indicated. 
„ 12. Fourth stage, in which two free thoracic segments are developed 
behind the head, and two or three soldered segments represent 
the pygidium. 
„ 13. Fifth stage, in which the thorax is longer than the head, and is 
composed of three movable segments and three soldered seg- 
ments in the pygidium. 
„ 14. Sixth stage, in which four free segments succeed the head, and 
three or four soldered segments form the pygidium. 
d n 2 
