THE FAUNA'OF THE ST. JOHN GEOUP. 163 



its growth. lu the latter respect, M.pulcliellus, of the Cambrian Basin at St. John, presents 

 instructive variation in this respect : when only a quarter grown, it has seven joints 

 corresponding to the full number of the adult in the older species, BI. Dawsoni, and also 

 to the complete number in the "Welsh form, M. punctatus, but when it reaches its full 

 growth, this Acadian form {M. pidchellus) has eleven rings in the axis. It seems fitting 

 therefore that this species should appear at a later time than M. Dawsoni. 



Passing to Group 2 of the trilobites in the above table, which group formed the 

 subject of a former communication to this Society,' we are among species which, although 

 still lacking in eyes, shew a difference of structure in so many respects beyond those of 

 Group 1 — as in the many joints of the thorax and in the presence of the facial suture — 

 that we cannot fail to see a great advance in structural development. This group is spe- 

 cially characteristic of the horizon whose fauna was studied by the late Prof C. F. Hartt 

 (Band c of Div. 1). One may trace in this group very marked changes in the form of the 

 cephalic shield from the embryonic to the adult stages. This capacity for variation indi- 

 cates the acquisition of powers of metamorphosis greater than those possessed by the 

 simpler forms of the first group. The investigator may also find in studying the embryos 

 and larva of this group, points of similarity to those of Groups 3 and 4, such as could not 

 exist in the more primitive genera of Group 1. 



Group 3 of the trilobites includes forms which, by their persistence in time 

 have become charactmstic of the Cambrian system as a whole, in which they differ from 

 those of Groups 2 and 4. As the trilobites of Group 3 extended so far onward in time, so 

 they were among the first that invaded the St. John Basin. But the pioneer species of 

 this group seem by their compact form and thick tests to have been specially adapted to 

 live in the sandy shallows of the ocean margin, and so they are naturally the first that are 

 found in these Cambrian beds. The minute species of Group 1, however, delighted in a 

 muddy bottom, and awaited the deepening of the waters on these shores before taking 

 possession of their heritage ; they may have been in existence in neighboring areas of the 

 ocean at the same time that the few known trilobites of Band b (undoubtedly of higher 

 organization) were living on these shores. 



By a comparison of the embryonic and larval forms of the trilobites of the third 

 group, the close family connection of these genera, especially the Ptychoparinse, may 

 readily be seen. Ellipsocephalus, though it differs from the Ptychoparinse in several 

 important respects, has been attached to this group for reasons which have been given in 

 a preceding part of this paper. 



Group 4 consists of species of those peculiar trilobites which, by all palœonto- 

 logists have been specially recognised as characteristic of this part of the Cambrian system. 

 These — the Paradoxides — differ so much, not only in their adult form, but also in their 

 stages of growth from the other trilobites of this formation, that if one were to associate 

 together the trilobites of our second and third groups in one, the Paradoxides would be 

 of equivalent value as regards their structural modification and their development. 



One peculiar feature in the development of Paradoxides is in the change of form of the 

 eyelobe. In the species of the St. John group which have been studied, the eyelobe in 

 the embryonic stage forms a continuous arch, extending from the posterior border on each 



' Trans. Eoy. Soc. Canada, 1884, Sec. iv. p. 99. 



