160 G. F. MATTHEW : ILLUSTRATIONS OF 



Sculpture. — The siTrface of the head and other parts in this variety exhibit a sculp- 

 turing similar to that of »S'. Acadica. 



Localily and Horizon. — Loose in fragments of dark grey shale at Lake Lomond, Simonds. 

 The shale contains also Microdiscus pulchellus, Hartt, and is therefore probably from some 

 part of Band d. 



Notwithstanding important differences between this and the typical narrow form of 

 S. Acadica, it seems desirable that it should stand as a variety of that species until it is 

 better known. In the general form of the head-shield, this species is not unlike S. holo- 

 metopa, Aug., but if Angelin's figure is to be trusted, that has rounded genal angles and 

 fewer joints in the thorax. If we assume that the pygidium, figured by Angelin as that of 

 S. brarhi/melopd, is really that of S. holomeiopa, a general resemblance between this species 

 and the above variety is manifest. 



Conclusions. 



In the examination of these ancient trilobites, we have now reached a point from 

 which we may with profit review this class of animals as they existed at the time when 

 the beds of Division 1 were deposited, especially as regards their relationship to each 

 other and as to the question of their evolution. To facilitate the consideration of these 

 points, the table (p. 161) has been drawn up, exhibiting the several fos.siliferous horizons 

 known in Division 1 and the supposed relationship of the different groups of trilobites 

 occurring in it. 



This table shews the range of the species, so far as known, at the two principal 

 localities in the St. John Basin. There is no occasion to refer to Band a, as this has not 

 yielded any organic remains. The fossils of Band b are few ; but, to exhibit the rich fauna 

 of Band c to advantage, the column containing it is divided by a single line, below which 

 (to the left) the species of the "lamellatus" bed are inserted and also those of the conglo- 

 merate-limestone, which in the eastern loi>ality (St. Martin's) lies at the base of Band c. 

 The species of Band d are placed at the left of the column representing it, because they 

 have not been traced upward through this section of Division 1, no fossiliferous exposures 

 of the higher beds having been found. 



Glancing at the left-hand column of this table one may observe that these trilobites 

 divide themselves broadly into four principal groups : — (1) Those which are devoid of 

 eyes and have a thorax of a few segments only ; (2) Those which lack eyes, but have a 

 long thorax ; (3) The smaller species possessing eyes ; (4) The Paradoxides. 



Of these groups, the first, in the light of the facts given in preceding pages res- 

 pecting the embryology of these ancient forms of life, is that yphich exhibits in the adult 

 the nearest approach to the larval forms of the more highly-developed species. And of 

 the two sections into which this group is divided, we cannot hesitate to give the 

 Agnosti the first place for simplicity of structure, and for the most perfect retention of 

 embryonic features. Among these features may be named the long glabella and the 

 thorax with two joints only. The younger tests in this group, so far as they have been 

 observed, shew little difference from the adult form. The genus is highly characteristic 

 of this early Cambrian period, as may be seen by the writings of Tullberg and Brogger, 

 who shew that a wealth of species characterises the lower part of the Paradoxides beds 



