154 G. F. MATTHEW : ILLUSTEATIONS OF 



To Mr. Hartt's description, which applies to the middle piece of the head-shield, I may 

 add the following : — 



Movable Cheek. — This is aculeate at the genal angle. Including the attached piece of 

 the fold in front, the cheek is three times as long as wide ; the marginal fold is about half 

 of the width of the inner part of the cheek at the middle, but at the posterior end eqvials 

 the width of the inner part. 



Tliorax. — This part of the body has fourteen (?) joints; twelve segments have been 

 counted in one example, but this is probably not the full number. The rachis is strongly 

 elevated and the backs of the rings rounded. The pleurœ are strongly geuiciilated about 

 half-way from the ring, and are traversed by a strong deep furrow, which narrows beyond 

 the geniculatiou. The tips of the first and second pleurre which scarcely extend beyond 

 the line of the dorsal suture of the head-shield, are rounded ; and in poorly preserved tests, 

 the succeeding segments seem to have rounded tips, but in well preserved examples, there 

 is a thin flexible extension of this part, terminating backward in. a sharp point. 



Pygidmm. — This is transA'^ersely lenticular. In the rachis are two rings narrow and 

 prominent, and two faintly defined ; the last terminates backw^ard in a bituberculate pro- 

 minence ; the rachis as a whole is broad and ijrominent, but does not reach the extremity 

 of the pygidium. Two furrows are visible on the lateral lobes, and the first is sufficiently 

 prominent, but the second is only faintly impressed. The rest of the lateral lobes is flat, 

 and rounded down at the margin. 



Sculpture. — The surface of the test in this species has a velvety look, owing to its 

 minutely roughened surface, which under a lens is seen to be minutely punctate. The 

 surfaces of the specimens preserved in slaty shale are marked also by small scattered 

 tubercles ; but as these do not appear on uncompressed specimens, it is supposed that they 

 are due to the impression of small canalets which perforate the test from the interior. 

 The inner side of the test is marked also by minute punctures, larger however and more 

 distinct than those which appear on the outer surface, and by scattered minute tubulations 

 that are just visible to the naked eye. The impression of the ocular fillet is plainly visible 

 on the inner surface of the cheek which is also graven with forking and inosculating 

 channels, especially in front of the ocular fillet. These markings vary much in distinctness 

 and are often readily seen behind as well as in front of the ocular fillet. They are visible 

 on the front slopes of both the fixed and the movable cheeks, radiating toward the anterior 

 marginal fold, which they may be seen to cross. As in the Conocoryphinne, they belong, 

 so far as has been observed, to the inner surface of the test. 



Narrow Form (Plate II, figs. 4 a to e.) 



Conocephalites Orestes Sç Hallii, Hartt, toe. cil. 



Ptychoparia Orestes Sç Hallii, Walcott, loc. cit. 



After a careful examination of the distinguishing marks of a form of trilobite found in 

 the shales of Div. 1 c, which the writer supposes to be that which Prof Hartt has desig- 

 nated Conocephalites Orestes and C. Hallii, he cannot arrive at any other conclusion than 

 that it is the narrow form of Solenopleura Robbii. It is found in such intimate and constant 

 association with this species, and so well represents its peculiarities, as they may be 



