146 G. F. MATTHEW: ILLUSTEATIONS OF 



point ofteu looks like a tubercle, but a close examination will shew behind it the line of 

 fracture marking the former presence of the spine. 



Movable Cheek (Plate II, fig. 1 b). — This part of the organism is briefly described by 

 Brogger, but is not referred to by Liuuarsson. Brogger's figure and description, however, 

 appear to apply to the narrow form. In the broad form, this part possesses a short, out- 

 turned genal point or spine, much shorter than the sharp, slender spine of the cheek of 

 the narrow form. The fold is strong and well rounded, and is nearly as wide as the inner 

 area of the cheek. The surface is finely punctate. There is a variety (Plate II, fig. 1 c), 

 having an arched row of hollow tubercles traversing the middle of the cheek length- 

 wise, and another row of more closely set tubercles just within the crest of the marginal 

 fold ; there are from six to twelve points in the inner row, and about twelve (with one on 

 the genal spine) in the outer row. These two rows may be looked upon as analogous to 

 the rows of spines, or minute tubercles on the two edges of a pleura, and as the anterior 

 row on the pleura is sometimes arranged in double rank outside of the geniculation, so 

 also does that on the marginal fold exhibit a similar tendency to form a double rank near 

 the extremity of the cheek. The genal spine itself in some examples is minutely and 

 closely denticulate on the upper surface. 



Thorax. — Our examples of the thorax are defective, but there is one shewing ten joints 

 of this part of the body, and the pygidium. This has many points of resemblance with the 

 small example figured by Linnarsson. From the proportionate length of the rings of the 

 rachis and the pleura, it is probable that this was the broad form ; in the Acadian 

 examples, the pleuras at the middle of the thorax are one and a half times the width of the 

 rachis, but in the narrow form about twice as wide. Dr. Linnarsson speaks of the pleurœ 

 being only slightly bent downward, and says that they are broadly rounded. These are 

 not characters of Ptychoparia, but his figure would give the impression that the pleurae 

 were broadly pointed; in our examples of the pleurœ of the broad form, they are bent at 

 an angle of 45°, but in the narrow form, the angle is much less. Some pleurre (as well 

 as movable cheeks) are devoid of spines, as has been observed by Linnarsson also to 

 be the case with the Swedish species. The rings of the rachis, as in the Swedish examples, 

 carry a row (sometimes several rows) of tubercles on the posterior ridge. In Acadian 

 examples of the narrow form, the central tubercle on the rachial ring is larger than the 

 others, and correspondingly the tubercles at the geniculation of a pleura are more promi- 

 nent than on other parts of the pleura. 



Pygidium (Plate II, fig. 1 d). — This part is lenticular in outline, and about twice as long 

 as broad. The rachis is prominent and marked by four rings besides the half ring at the 

 front. All the rings are somewhat enlarged at the ends, and the anterior pair are sharp 

 and elevated at the middle ; the foremost ring is decidedly longer and heavier than the 

 second, and has a faint lunate furrow on the posterior side ; the third ring is faintly 

 marked off from the fourth, and is somewhat enlarged at the ends; in some examples the 

 central part of this ring is depressed, and the ends of it appear to be small tubercles (Plate 

 II, fig. 1 e) ; the fourth ring is almost always bituberculous at the ends as Brogger describes 

 the species. Behind these tubercles, the rachis descends rapidly to a subangular point at 

 the extremity of the pygidium. The side lobes of the pygidium have two concave ribs on 

 each side, that are distinct, and a third, short and faintly (sometimes not at all) defined, 

 which is strongly turned backward : the concavity or furrow in the two anterior ribs is 



