FROM THE CARBONIFEROUS FORMATION. 175 



leading to the presumption, that, if better displayed, the creature would show the usual 

 appearance of a swollen second fourth of the body. The dorsal plates are very much 

 larger in the front than in the hind part of the animal, and are nearly quadrate or even 

 slightly broader than long (as exposed), while in the middle they are of equal length and 

 breadth, and posterioidy are longer than broad. This refers however only to the plates as 

 they are shown above the line which appears to separate, along the side of the body, the dor- 

 sal and ventral plates ; but in the hinder third of the body, or the last dozen segments, one 

 sees far below this line the true rounded lateral edges of the segments ; between the two 

 very different margins the ventral plates appear, and continue forward nearly to the head, 

 with occasional indications of the division line between consecutive dorsal plates seen 

 through them, or through which the ventral plates are seen ; as in many other fossils, 

 both carboniferous and tertiary, the sutural marks of both an originally underlying and an 

 overlying chitinous mass appear upon the same surface, so as frequently to render it quite 

 uncertain which was originally superincumbent. Judging from these appearances the 

 dorsal plates, perhaps only when flattened, were four or five times broader than long, and 

 in front of the last six segments regularly and fully rounded ; in these last six segments, 

 the anterior half is rounded as before, or very nearly so, but the outer hinder angle is pro- 

 duced, bearing a triangular process which extends to the middle of the succeeding segment ; 

 together they give a straight margin to the sides of the body at this point, and evidently 

 form by their combination a terminal flap, since the triangular process closes the lateral 

 excavation which the rounded front angle would otherwise create (whence the specific 

 name); a rapid forward and backward movement of this part, after the manner of macruran 

 Crustacea, would propel the creature backward in the water; and we have seen that the 

 structure of these myriapods allowed so much freedom of movement between the 

 joints, as to render it no great surprise to find a movement so peculiar for myriapods to- 

 day indicated by the special structure of the segments. It adds too another fact in support 

 of the theory that these were aquatic or partially aquatic animals. 



Perhaps a similar flexibility of the body is indicated by a feature seen in the ventral 

 plates, which seems entirely different from anything hitherto found in the Archipolypoda. 

 These plates, as stated, are visible along the inner side of the body throughout a large part 

 of its length, two to each one of the dorsal plates ; and along the middle of their course they 

 are broken by a longitudinal suture, (i. e. transverse to the segment), which is only not con- 

 tinuous from one plate to the next on account of the lateral sliding, due to the curled posi- 

 tion of the animal ; where it becomes straight, at the tail, these breaks are also continuous ; 

 in one instance, near the middle of the body, the ventral plate is again broken by a second 

 suture next the dorsal plate, but no similar case is noticed elsewhere. Such a fracturing 

 of the ventral plates has nowhere else been seen in these ancient myriapods from Mazon 

 Creek, although in several the parts equivalent to these are amply exposed ; but their 

 regularity here is such that it cannot be looked upon as accidental, but only as an 

 inherent structural feature, and reminds one of the repeated and regular fracture of the 

 dorsal plates in Xylobius, where I have shown this peculiarity to be a feature of the 

 entire genus. 



Next the outer side of the coiled specimen one sees, partly on one stone, partly on its 

 counterpart, a partial duplicate as it were of the fossil, a feature which I have seen in 



