THE FAUNA OF THE ST. JOHN GEOUP. 147 



broad and open, but does not reach the margin ; that of the first rib is curved backward at 

 the extremity. 



Sculpture. — The elevated edges of the rings and of the lateral lobes, are armed with 

 rows of very minute tubercles, similar to those of the pleurte. 



Length, 2| mm. Width, 6 mm. 



Four joints is the normal number in the rachis of the pygidium of the broad form or 

 type of this species, as five is iu the narrow form. There are two strongly marked ribs 

 in front where the pleurae overlap, but the difference between the two pygidia is more 

 observable in the posterior rings of the rachis ; this difference which exists between the 

 pygidia of the two forms probably accounts for the uncertainty which, according to 

 Linnarsson, attaches to the number of joints in the pygidiiim of this species. In the great 

 majority of the Acadian individuals of this species, the end of the rachis has the bituber- 

 culate termination described and figured by Brogger, but which Linnarsson could not 

 find in the Swedish examples ; in some Acadian pygidia, the last ring but one also exhibits 

 a pair of tubercules, less prominent than the posterior pair (Plate II, fig. 1 e.) The end 

 of the rachis is more obtuse in the broad than in the narrow form, and indeed some pygidia 

 appear to have the rachis abruptly cut off behind the tubercles ; but in the more normal 

 form, it possesses a depressed, faintly-outlined extension to the posterior margin. The 

 lateral costse and furrows do not reach the margin, and are broadly (uot sharply) and 

 moderately (not deeply) impressed, as in the narrow form ; the pygidium differs also from 

 the narrow form in that the third costa and furrow are more obscure. There is also a 

 difference m the proportions of the pygidia of the two forms : in the narrow it is, as des- 

 cribed by Linnarsson about twice as wide as long, but in the wide form the width is con- 

 siderably more than double the length. 



Horizon and Locality. — This species occurs in the dark grey shales of Div. 1 c? at St 

 Martin's in company with Paradoxides Abenacus, Microdiscus pulchellus, etc. 



Narrow Form {var. alata.) (ri. II, figs. 2a to/.) 



This the writer supposes to be represented by a trilobite whose remains are abundantly 

 preserved in the same shales which carry the species above described as the broad form of 

 P. Linnarssoni. It is more commonly met with than the broad form, and possesses certain 

 characteristics by which it may readily be separated from it. The anterior marginal fold 

 of the centrepiece of the head shield is longer, straighter and narrower ; the glabella is 

 longer and more conical ; the occipital ring is narrower and bears a longer spine ; the free 

 cheek is wider and bears a sharp, flaring, genal spine ; the pygidium has one (sometimes 

 several) more joints iu the rachis, which is narrower and longer than that of the broad 

 form ; the side lobes of the pygidium have more distinct costse ; the test in addition to the 

 minute punctures, has faintly marked, scattered tubercles, and is thinner and more easily 

 flattened than that of the broad forms. 



Movable cheek (PI. II, fig. 2 b). — When full grown, the free cheek of this form of the 

 trilobite is easily distinguished from others occurring with it by the greater width of the 

 inner area, and by its long sharp spine. The marginal furrow, narrow in front, attains 

 greater breadth and depth toward the genal angle. The course of the suture in front of 



