KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 34. \:o 8. L5 



prominent and definite from the latera] appendages, which have become narrow, thougfa of 

 the sninc length as before. The two firsl centra] segments seem t<> have been nnited into one. 



Between the hypothetica] figure l and fig. 5, there is evidently a greal lacuna, 

 not yet filled up. In the interva] of time the two appendages, which we saw in fig. I, 

 have been much modified, the posterior one having los) si» much in bulk, and the 

 .-interiör being lengthened and stretching oui beyond the börder of the shell in a 

 narrow spine alongside the posterior one. The centra] segmented pari bas nov assu- 

 rned a shape, which on comparing it with the following stages of developmenl makes 

 its true nature evident and that it indeed is what in the aduH animal becomes the 

 glabella. The nieaning of the previous stages then also is easily understood. The 

 central segmental piece in thein is the glabella or we may, as Bernard already has 

 done, call it for rhachis and the side appendages for pleura, as this lit-tle larva represents 

 the whole body of the future trilobite, and embodies all its parts in nuce. Through 

 the great changes which these pleura undergo, it results, as we have scen, that two pair 

 vanish, being incorporated with the large fourth pair and that only two rest for a while, 

 the anterior one being the so much renowned frontal-lob or eye lobe and the second one 

 the so called »ornamental spine», which in fact is a compound of the original second 

 third and fourth pleura of the corresponding segments. 



It is to be borne in mind that this larva, which represents the future trilobite in 

 its earliest stage, is nothing but the head, or what in the adult takes the place of the 

 head, and especially its dorsal surface and that it thus solely consists of the future 

 head. 



In the figure 5 (Ford's fig. 1.) there are the first signs of the pygidium coming 

 and in the fig. 6 it is well developed, but the thorax is still non apparent. In the stage 

 fig. 6 both pleura have increased in length and the compound one also in bulk. They pro- 

 ject with spiny points beside the beginning pygidium and the anterior pleura have united 

 across the first segment of the glabella through a narrow ridge, which seems to cut that 

 segment in two. The sequence in the order of development or growth thus is first the 

 head, then the pygidium and last the thorax. At least it is so in these the oldest of 

 all trilobites. But in nearly all trilobites of which there are good data, the head is the 

 part first developed. 



Evidently a large hiatus exists between the stages represented in tigs. 6 and 7 

 (Ford's figs. 2 & 3, 1877), in the latter of which the animal, though not adult, has had 

 the thorax and pygidium added to the head. The modifications in the size of the pleura 

 are the chief changes. The anterior pair is reduced and retired within the posterior börder 

 of the head forming a semilunar arch joined with the occipital ridge in an angular bend. 

 The posterior pair is enlarged and its spine is by and by reduced (figs. 8, 9, 10) till it 

 quite disappears and only the wide semicircular field between the first pleuron and the 

 glabella is left behind as a remnant of its dilated body. It is to be remarked, that 

 while in the plurality of the adult Olenellidie all traces of the spiny projections of the 

 second pleuron have vanished, they are still retained in the adult Holmia Kjerulfi, though 

 not in the American Holmine, and thus give it at the same time a more ancient and a more 

 larval stamp. The shallow groove along the back of its first pleuron indicates strongly 



