THE FAUNA OF THE ST. JOHN GROUP. 149 



At the first glance it might be thought that the narrow form of this species was 

 Angeliu's Anomocare microphthalmum, especially in examples where the spine has been 

 detached, but if one notes the shorter anterior limb of the cheeks, the shorter posterior 

 angles of the middle piece of the shield and the longer and more cylindrical glabella, it 

 will be seen that they are different. The two species also occur at different horizons, for 

 while the Acadian trilobite is associated with the American representative of Paradoxides 

 Tessini, Angelin's species above named is from the horizon of P. Forchammeri. 



Development of the Young. 



Great numbers of the head-shields of young individuals of this species are sometimes 

 found on layers of the shale in company with Mkrodiscits pidchellus, Agnost/is declivis, etc. 

 They are worthy of study, as showing the great changes in the proportion of the parts 

 during growth. 



1st Stage (PI. II, fig. 1/). — In its first recognisable stage (as a trilobite) this species pre- 

 sents an oval test in which a very narrow glabellar ridge, having a club-shaped, faintly- 

 raised front is developed ; this ridge is long, cylindrical and sharply elevated above the 

 general surface of the test ; sometimes this club-shaped front is not visible, and the embryo 

 appears to be marked only by a sharply defined longitudinal or axial ridge extending 

 across the middle three quarters of the oval body, and abruptly terminated at each end 

 considerably within the margin of the body; in others, a faint transverse ridge is found 

 at the extremity of the embryo opposite to that at which the club-shaped anterior end 

 of the glabellar ridge is subsequently developed ; this transverse line is the posterior 

 margin of the future head-shield. Other examples exhibit faint transverse depressions on 

 the axial ridge itself, which from their position may be regarded as incipient glabellar 

 furrows. 



2nd Stage (PI. II, fig. 1 g). — In this, a change in the outline of the embryo takes place, it 

 becomes more elongated, and assumes an oval outline, owing to the narrowing of the front ; 

 the club-shaped front of the glabellar ridge is now plainly A'isible, and the impressions on 

 the sides of the ridge.where the glabellar and occipital furrows are subsequently developed, 

 are more distinct. The position of these glabellar impressions is remarkable, and exhibits 

 the rudimentary condition at this stage of the somites contained in the cephalic shield ; 

 both the occipital and the two posterior glabellar furrows are crowded together at the back 

 end of the axial ridge, while the anterior furrow, now relatively more prominent than at 

 later periods of growth, is far distant from the others, near the anterior end of the glabellar 

 ridge ; it is somewhat more than a quarter of the length of that ridge from its anterior end. 

 There is therefore at this stage of growth a wide space on the glabellar ridge between the 

 anterior and the two posterior furrows. A pair of minute and not very distinct tubercles 

 is placed on the sides of the glabellar ridge just in front of the indentations that mark the 

 place of the anterior furrow ; they are on the highest part of the front of the glabellar 

 ridge and are possibly rudimentary organs of vision, but the ocular fillet which appears at 

 a later stage, does not seem to diverge from the glabella at this point, but at a point a 

 little in front of it. 



At this stage the occipital ring is distinctly defined, as also the posterior marginal 

 furrow, as well as the suture where the separation of the head-shield and pygidium 



