THE FAUNA OF THE ST. JOHN GEOUP. 151 



The second furrow (not seen in the example figured, but in a number of others) is also 

 advanced nearly to its permanent position, while the anterior furrow (now comparatively 

 obscure) by the condensation of the front of the glabella, also comes near to that part of 

 the front of the head. By these changes, the relative importance of the different segments 

 of the body, marked off by these furrows, is entirely changed ; the anterior, which is a 

 sensory segment, is reduced, while the next two, which are connected with the mandu- 

 catory organs, are greatly increased in importance. The remarkable changes of form at 

 this stage are probably connected with a change in the habits of the organism, the 

 somites connected with the alimentation of the animal being adapted for more active use. 

 At this stage also, the eyelobe on each side of the shield is always clearly visible, and in 

 some examples is seen to be connected with the front of the glabella by a distinct ocular 

 fillet. In this test and its companions of like size, a slender occipital spine is developed, 

 the anterior marginal fold is distinctly marked, and there is usually also a narrow, 

 raised band connecting the front of the fixed cheeks, and separated from the anterior fold 

 by a furrow. The Agrauloid features of this head-shield are worthy of remark. And as 

 the enlargement of the posterior glabellar somites appears to exhibit clear indications of 

 the power to take and utilise food, this may be regarded as the first larval stage of the 

 trilobite. 



6th Stage (PI. II, Fig. 1 I). — Many examples of the head-shield at this stage, and a few 

 of the corresponding pygidium occur in the shales which contain the larger tests of this 

 species ; they are often wider in front than the example figured, but otherwise much the 

 same. The cylindrical form of the glabella is still fairly preserved, but the head is more 

 condensed, the ocular fillets are now c[uite distinct on the mould (represented in the 

 figures) and sufiiciently marked on the outside of the test. The raised band in front of 

 the glabella which connects the cheeks is somewhat wider, and the outlines of the head 

 at the sides have been changed by the withdrawal of the eyelobes toward the sides of the 

 glabella, in connection with the enlargement of the movable cheeks. 



Gth Stage (PI. II, Fig. 1 m). — The head-shield at this stage does not differ much from 

 the adult form, but a trace of its immaturity is seen in the slight obliquity of the furrows 

 at the posterior end of the glabella (the oblic^uity being greater in the test of the adult), 

 and also in the comparatively narrow space between the first glabellar furrow and the 

 occipital ring. 



In the stages later than those described, the head-shield differs but little from that of 

 the adult ; the most notable change of form observable, as the test approaches the adult 

 size, is the somewhat shorter and more conical form of the glabella, and the fixing of the 

 position of the furrows ; the anterior furrow being as far from the posterior at the side of 

 the glabella as that furrow is from the base of the glabella. 



G' 



Development of the Young in the Narrow Form- 

 No perceptible distinction is found between the broad and narrow forms, by which 

 they can be separated at an earlier stage than the second larval stage, the fifth from the 

 embryo (PI. II, fig. 2 d). In tests of this age, the distinction is apparent, in the broader 

 front of the shield, the broader limb in front of the glabella, and in the proportionately 

 longer glabella of the narrow form. At this stage, the narrow form has not been found to 



