1871.] WOODWARD XIPHOStTRA. 51 



EXPLANATION OF ILLTJSTEATIONS. 



Figs. 1-8. Trinucleus ornatus, Sternb. sp. (copied from Barrande's ' Systeme 

 Silurien du Centre de la Boheme," Prague, 1852, 4to, plate 30). Speci- 

 mens arranged in series according to their supposed age. (All the stages 

 figured by Earrande are not given here.) 



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 de- 



veloped. 



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 thoi-acic segments and fully developed genal 



spines. 

 9-15. &o Airs»,;fa, 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. 



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 indications 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 thoi'acic 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 com- 



posed of three movable segments and three soldered segments in the 

 pygidium. 



14. Sixtli stage, in which four free segments succeed the head, and three or 



four soldered segments form the pygidium. 

 16. Tenth stage, in which eight free segments succeed the head, and three 

 soldered segments form the pygidium. [In the twentieth stage figured 

 by Barrande the adult has seventeen free thoracico-abdominal segments 

 and two soldered ones (the pygidium).] 



16. Egg of LimiUus poIyphemus : a, the chorion ; b, the exochorion (after 



Dohrn). 



17. Third stage in the embryo of Linmlus : a, chorion ; b, exochorion (after 



Packard). 



18. Fourth stage (?) in the embryo of Limulus (after Dr. Packard's figure). 



19. Fourth stage (?) in the embryo, of Limv.lus : 1, antennules ; 2, antenna ; 



3-6, maxillipeds ; 7 and 8, thoracic plates afterwards bearing the 

 branchias ; m, the mouth ; x, the ovarian apertures (?) ; a, the abdomen 

 (after Dohrn). 



20. Fifth stage (?) of embryo oi Limulus (after Dohrn). At this stage the 



exochorion is split, and the chorion is expanded by the admission of 

 water by endosmose, in which the embryo is seen to revolve. 



21. Ninth stage (?) of embryo, "just before hatching" (after Packard): 



dorsal aspect. 



22. The same : side view of embryo. 



23. Larva of Limulus recently batched (after Packard). 



24. Larva of Limulus on hatching (the " Trilobitenstadiu'in " of Dohrn). 



e2 



