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Placed beside the cast of the great Styionurus Scoticus,* it did not require 

 a very protracted examination or comparison to demonstrate most clearly that the 

 Rowlestone fossil had no relation to that genus ; but this point arrived at, I 

 found myself for some time quite unable to decide what its real affinities were. 



The ornamentation of the dorsal surface differs greatly when compared 

 with Styionurus Scoticus. 



Assuming it to have belonged to that species, as I at first supposed, it could 

 only have been one of the abdominal segments. 



The dorsal surface of these in Styionurus is covered with minute scale- 

 markings, whilst a row of pear-shaped tubercles borders the posterior margin of 

 each segment. 



These abdominal segments moreover are furnished, in Styionurus Scoticus, 

 with large epimeral pieces increasing in size as the segments diminish in breadth 

 backwards to the telson. Lastly, they are destitute of appendages of any kind. 



In the Rowlestone specimen (Sketch No. 9, Fig. 1. A.) the dorsal surface 

 of the segment is almost covered with prominent rounded tubercles disposed 

 irregularly over its surface, a fine median line is also seen dividing the segment 

 down the centre into two equal parts. From this line, near the middle of the 

 segment, a transverse depression spreads out on either side destitute of tubercles 

 but marked on either hand by a smooth rounded depression, about the size of a 

 pea, and f ths of an inch from the median line. 



The anterior border, which i3 rounded, and, as it were, bevelled off. is quite 

 smooth for 1$ inches, save that it is marked by a slightly raised transverse line 

 or ridge which crosses it at I, I, and by two small sub-central depressions. 



This is evidently the portion of the segment which was overlapped by the 

 preceding somite. The lateral borders are destitute of any epimera, a fact, 

 which taken in connection with the bases of paired and jointed appendages (to 

 be presently noticed) on the underside, strongly suggests the ambulatory rather 

 than natatory habits of the animal of which the fossil-remains under considera- 

 tion formed a part. Assuming that we have nearly the whole of one segment 

 seen in Fig. 1 A. the breadth of this segment is 4 inches, and the total length 3A 

 inches, of which about 2^ inches were exposed, the remaining 1 inch being 

 inserted beneath the hinder border of the anterior segment. 



An inspection of the ventral aspect of the same fossil reveals to us, a 

 structure unlike that of any palasozoic Crustacean heretofore met with (See 

 Sketch No. 9, Fig. 2 B.) but peculiarly insect-like. "We see in the centre a narrow 

 sternal piece (s.) deeply infolded down the median line, from which pairs of 

 limbs take their rise (n, n ; o, o). Of these limbs, only the strongly-arched and 

 flattened basal joints, with fragments of the 2nd articulation remaining at n, n, 

 are preserved. 



* The original of which still remains in the Museum of the discoverer, James Powrie, 

 Esq.. F.G.S., of Keswallie, Forfar. 



