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 Mesonacidae, Soleno- 

 pleuridas, and Olenidse, and a glance at Barrande's figures of "flydrocephalus" carens and 

 "///' saturnoides, 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 Mcsonads 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 

 Dalmanitcs and Cydopyge 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 Dahnanites 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 i 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. 



