THE FAUNA OF THE ST. JOHN GROUP. 107 
eyelobe than in front of the glabella (2 mm.) so that, if the addition to the glabella was made 
in the way supposed, while the vertical height of the glabella was increased in all parts, 
the greatest gain would have been in the middle. 
In P. lamellatus, Hartt, var. loricatus, the vertical plates occur at regular intervals along 
the slope of the dome of the glabella as though they marked regular stages of growth: and 
the raised lines on the slope of the dome of P. eteminicus and its allies may indicate the 
transfer of narrow zones of the head shield to the glabellar area, and those on the anterior 
marginal fold, of similar belts to the rim of the shield; for the lines on the latter are not 
parallel to the inner but rather to the outer margin; on that side, also, the lines are more 
crowded, as though the fold were growing wider and more compact at the expense of the 
flat area. Whether there was any such process of condensation of the test between the 
periods of moulting or not, it is clear that there was a gradual change in the proportions 
of different parts of the shield during the advance from youth to maturity. 
It is remarkable that the occipital spine in many of the Saint John forms of the Paradoxi- 
des should become reduced, and in some cases should disappear altogether in the later stages 
of growth. It is present in the younger stages of all the species and varieties, and its gradual 
reduction in size accompanies the condensation of the occipital ring. Originally it appears 
to have been of more importance, and perhaps with the ring served as a protection to the 
posterior immature extension of the body of the trilobite. Spence Bates, who has studied 
the early stages of the common shore crab (Carinus menas), observed the appearance of a 
prominent dorsal spine in the second zoéa stage of that crustacean. The great prominence 
of this process at so early a stage of Carinus reminds one of the importance of the occipital 
ring and spine in the early stages of the St. John Paradoxides. 
Another feature of growth which the examination of the tests of these Cambrian 
trilobites has revealed is the rapid changes which occurred during the earlier stages of 
growth. Development did not always proceed at a regulated pace. For instance, the 
surface of the dome of the glabella of P. lamellatus, Hartt, within the rows of plates, pre- 
sents quite a different appearance as regards the surface markings, to that which later 
marks the plated slopes of the dome. So also in the tests of P. acadicus, the 10.9 mm. 
size, has many embryonic features that are wanting in the next size: and the arrest in the 
backward movement of the fourth furrow of the glabella of the 15.4 mm. size (if net 
earlier) in the variety breviatus (of P. eteminicus) is also significant of an early maturity in 
that form, with which other features of this test agree. In the more complete series of 
P. eteminicus, it is plain that the most rapid and vital changes occurred in this species, in 
the earliest stages of growth, and that the development in the later ones was slower, and 
was arrested in one point after another as the trilobite approached maturity. Rapid 
change of form and the earlier stages of growth and development on the lines indicated 
above, characterized the Saint John Paradoxides; just as is in Sao hirsuta and Olenellus 
asaphoides corresponding changes in the process of growth fixed the distinguishing features 
of those two species. 
