ISO G. F. MATTHEW: ILLUSTRATIONS OF 



subseqvieutly takes place. The cheeks are tumid aud well rounded ou the outer slopes, 

 but at the forward eud they are separated by the glabellar ridge. 



The pygidium is already like that of the adult in outline, but the rachis is not strongly 

 raised, aud the lateral costae are scarcely visible. The posterior end of the embryo is much 

 more flattened than the anterior, and the latter is not only elevated, but is round and 

 tumid. 



The features above described are not altogether constant, for in some embryos the 

 anterior enlarged end of the glabellar ridge is not defined, but a sharply impressed line 

 terminates the anterior end of the glabella, and the side lobes of the embryo are con- 

 tinuous around its front. The separation of the head-shield and pygidium is not always 

 as well marked as in the example figured, for sometimes it is visible on one side of the 

 embryo, but not on the other. 



'èrd Stage (Plate II, Fig. 1 h aud /). — Embryos at this stage are more plentiful than at 

 the preceding. The head-shield and pygidium are now found to be separated. The 

 former has changed in form, having become more quadrate in front and wider, the width 

 and length being nearly equal. The glabellar ridge exhibits important changes : it is wider 

 in proportion than at the earlier stage, and the posterior furrow is only about three times 

 as far from the anterior furrow as from the occipital furrow. The two posterior furrows 

 are transverse, and not deflected backward as at later stages ; there is a faint indication of 

 a marginal furrow in front of the glabellar ridge. Seen from the front, the glabellar 

 ridge appears to be pyramidal (Plate II, fig. 1 i), and sweeping off from it on each side 

 to the lateral margins two faint ociilar fillets may be detected. It is doubtful if the eye- 

 lobe, in all cases, is developed at this stage, though there are indications in the outline 

 of the lateral margins that it is sometimes present ; when visible, it is seen to be very low 

 down on the side of the test. 



This stage in the Ptychoparinse presents many points of resemblance to the adult in 

 Aguostus. It is that at which the segments of the thorax first appear, and it is still 

 marked by the proximity of the two posterior furrows of the glabellar ridge to the occi- 

 pital ring, and by the prominence of the anterior furrow. In the genus Agnostus, the 

 glabellar furrows, except the anterior one, are more or less obscure, or obsolete, and the 

 occipital ring appears to be suppressed ; the prominences known as the basal lobes in 

 Agnostus answer to the posterior lobes of the glabella in the higher trilobites, hence it 

 will be seen that in this genus the posterior furrow is thrust back to the base of the 

 glabella, as in the embryonic stages of Ptychoparia; and that the anterior of the three 

 glabellar furrows, forming, as it frequently does in Agnostus, a strong transverse depres- 

 sion, possesses the prominence which we observe in the rudimeut of that furrow in the 

 initial stages of Ptychoparia and its allies. 



Uh Stage (PI. II, Fig. 1 k). — The trilobite here enters upon a stage in which the 

 outlines and features of the mature test begin to appear. The glabella has become 

 considerably shorter, and the club-shaped anterior end has disappeared ; it is much wider, 

 has parallel sides, and is rounded at the anterior end. A remarkable change in the posi- 

 tion of the glabellar furrows has taken place ; the outer ends of the posterior furrow, in 

 place of being near the back end of the glabella, are now, in some individuals, nearly 

 halfway to the front, but though the ends are so far forward, the point where the furrow 

 crosses the axis of the glabella is not far from its original position in the younger test. 



