Stage Length in Percentage Segments in Segments in mm. of pygidium thorax pygidium ======================================================== 1 0.66 30 0 2 2 0.75 33-1/3 0 3 3 1.00 46 0 5 4 1.50 32 2 5-6 5 1.50 25 3 4 6 1.75 23 4 4 7 1.80 21 5 3 8 2.00 17 6 3 9 2.50 13 7 3 10 3.00 12 8 3 11 3.50 11 9 3-4 12 4.00 11 10 3-4 13 5.00 10 11 3 14 5.50 9 12 2-4 15 6.00 8 13 3-4 16 6.50 8 14 3 17 7.00 7 15 3 18 7.50 7 16 3 19 7.50 6 17 2 20 10.25 6 17 2 This table shows the rapid increase in the length of the pygidium till the time when the thorax began to be freed, the very rapid decrease during the earlier part of its formation until six segments had been set free, and then a more gradual decrease until the entire seventeen segments had been acquired, after which time the relative length remained constant. From an initial proportion of 30 per cent, it rose to nearly one half the whole length, and then dwindled to a mere 6 per cent, showing conclusively that the thorax grew at the expense of the pygidium. If this conclusion can be sustained by other trilobites, it indicates that the large pygidium is a more primitive characteristic of a protaspis than is a small one. I have already shown that the pygidium is proportionately larger in the protaspis in the Mesonacidæ, Solenopleuridæ, and Olenidæ, and a glance at Barrande's figures of _"Hydrocephalus" carens_ and _"H." saturnoides_, both young of _Paradoxides_ will show that the same process of development goes on in that genus as in _Sao_. There is first an enlargement of the pygidium to a maximum, a rise from 20 per cent to 33 per cent in the case of _H. carens_ and then, with the introduction of thoracic segments, a very rapid falling off. All of these are, however, trilobites with small pygidia, and it has been a sort of axiom among palæontologists that large pygidia were made up of a number of coalesced segments. While not definitely so stated, it has generally been taken to mean the joining together of segments once free. The asaphid, for instance, has been thought of as descended from some trilobite with rich segmentation, and a body-form like that of a _Mesonacis_ or _Paradoxides_.