THE PROTASPIS. 135 
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 forma- 
tion 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 pygid- 
ium 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 Mesonacidse, Soleno- 
pleuridse, and Olenidse, and a glance at Barrande's figures of "Hydrocephalus" carens and 
"H." satumoides, both young, of Paradoxides. will show that the same process of develop- 
ment 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 palaeontologists 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 seg- 
mentation, and a body-form like that of a Mesonacis or Paradoxides. 
The appeal to the ontogeny does not give as full an answer to this question as could 
be wished, for the complete life-history of no trilobite with a large pygidium is yet known. 
While the answer is not complete, enough can be gained from the study of the ontogeny of 
Dalmanites and Cyclopyge to show that in these genera also the thorax grows by the break- 
ing down of the pygidium and that no segment is ever added from the thorax to the pygid- 
ium. The case of Dalmanites socialis as described by Barrande (1852, p. 552, pi. 26) will 
be taken up first, as the more complete. The youngest specimen of this species yet found 
is 0.75 mm. long, the pygidium is distinctly separated from the cephalon, and makes up 25 
per cent of the length. This is probably not the form of the shell as it leaves the egg. At 
this stage there are two segments in the pygidium, but they increase to four when the test 
is 1 mm. long. The cephalon has also increased in length, however, so that the proportional 
length is the same. The subjoined table, which is that compiled by Barrande with the pro- 
portional length of the pygidium added, is not as complete as could be desired, but affords 
a very interesting history of the growth of the caudal shield. The maximum proportional 
length is reached before the introduction of thoracic segments, and during the appearance of 
the first five segments the size of the pygidium drops from 25 to 15 per cent. Several 
stages are missing at the critical time between stages 8 and 9 when the pygidium had added 
three segments to itself and has supplied only one to the thorax. This would appear to 
have been a sort of resting or recuperative stage for the pygidium, for it increased its 
own length to 20 per cent, but from this stage up to stage 12 it continued to give up seg- 
ments to the thorax and lose in length itself. After stage 12, when the specimens were 
8 mm. long, no more thoracic segments were added, but new ones were introduced into 
the pygidium, until it reached a size equal to one fifth the entire length, as compared with 
one fourth in the protaspis. 
