156 G. P. MATTHEW: ILLUSTRATIONS OP 



of the species of both genera ; but at the 3 mm. size, the characters of the species are 

 clearly indicated, though the whole shield, and especially the glabella, are uarrrower 

 than in the adult. In head-shields that are less than one mm. long, a peculiarity in the 

 short depressed anteiior marginal fold in the species belonging to Liostracus will often 

 enable the observer to separate the two, and even in those of less size, the deeper dorsal 

 furrow and more spreading cheeks of Solenopleura are sometimes observable 



1st Stage (Plate II, fig. 3g.) — The most elementary stage recognised is one in which 

 the outline of the body is nearly circular (broadly obovate), with a surface sculpture of a 

 cruciform ridge. On closer examination this ridge is seen to be formed by the rachis of 

 the body and a tranverse furrow marking the junction of the parts which in the next stage 

 separate to form the head-shield and pygidium. The only distinct furrow on the rachis 

 at this period is the one which forms the dividing line between the two parts of the body 

 above named. By close inspection, however, one can see undulations of the surface of 

 the rachis shewing where the glabellar furrows will come ; and the same peculiarity is 

 observable in this as in the corresponding stage of Plychoparia Limmrssoni, viz., that the 

 posterior and occipital furrows are close together, and both near the posterior end of the 

 glabellar ridge ; the two anterior furrows are more widely separated and are near the 

 anterior end of the glabellar ridge. The side lobes of the cephalic portion of this embry- 

 onic test are smoothly rounded, without any indication of eyelobes or ocular fillet. 



The pygidium is outlined by a pair of transverse furrows, which do not extend to the 

 margin ; and on the rachis are the faint impressions of two furrows destined to mark off 

 the two anterior rings of the rachis ; the rachis does not extend quite to the posterior end 

 of the body, and no costœ or furrows are visible on the lateral lobes. 



2nd Stage (Plate II, fig. 3/(). — In this, the head-shield and pygidium are usually 

 separated. The former exhibits in the rachis embryonic features similar, for the most part, 

 to those of the earlier stage just described, except that all the segmentations of the rachis 

 are more distinctly defined, and there is a narrow, posterior marginal fold. The glabellar 

 ridge has become more cylindrical, but the posterior furi'ow is still close to the occipital. 

 The latter furrow is much more distinct than in the preceding stage, and the posterior 

 marginal furrow, though narrow, is strongly indented. No eyelobes are as yet Aisible, 

 but there is a faint ocular fillet, which originates near the anterior end of the glabellar 

 ridge, and descends with a curve to the lateral margin. Its position is indicated on the 

 front of the shield, but it cannot be seen from above, owing to its very slight relief above 

 the general surface of the test. 



Srd Stage (Plate II, fig. Si). — At this stage the growing trilobite assumes the larval 

 form, but (in the individual figured) not fully, as the posterior furrow is still close to the 

 occipital ring, has A'ery little obliquity, and is only faintly impressed, but is nearly equally 

 distinct all across the glabella. In other respects, however, this test exhibits many 

 characters pointing toward the adult form. Both an anterior marginal fold and a faint 

 representation of the anterior limb of the fixed cheeks are visible, so that the rachis no 

 longer extends to the anterior end of the body, but the glabella is distinctly defined ; this 

 part however is long and cylindrical and very unlike that of the adult in its form. The 

 backward-projecting and leaf-like occipital ring now shews a tubercle on its surface, but 

 this is much closer to the posterior margin than it is in the full-grown test. With the 

 growth of the anterior marginal fold, and the development of the ocular lobes, a change in 



