BEITISH FOSSILS. 



3 



just before it again suddenly expands to form the broad truncate tip ; 

 the front edge is very slightly sinuous. The points of the front angles 

 are broken off, but they appear to have been produced laterally rather 

 than decurved, in which they differ from those of fig. 6. A narrow, 

 but distinct sulcus all round distinguishes the central more convex 

 portion, which is straight at the upper or basal margin, but semi- 

 circular in front. A slight prominence rises on the middle of the 

 upper margin, on each side of which strong imbricating strise 

 descend and meet V fashion along the median line. They extend 

 only along the upper edge, and beneath them fine vertical striae 

 arise, and cover nearly all the rest of the central space. Concentric 

 striae run round all the margin, both on the front and sides, and the 

 latter are also marked by a flexuous keel which foUows the curve 

 of the lateral notch. 



Fig. 6, a specimen from Waterford, where the R. dorsospinifer 

 occurs with it, difiers very little in size or outline ; it has the flexu- 

 ous keel and notch on the sides, and the expanded angles of the base 

 and apex. But the latter, instead of being extended outwards (as 

 they appear to do in fig. 5), curve down and forwards, forming short 

 falcate lobes 6, b, — ^the front margin is a little more sinuous. The 

 specimen is more compressed, and the central portion therefore less 

 convex, though of the same shape. The large imbricating strise too 

 are not preserved. The diflferences above pointed out are, however, 

 very trifling ; and we believe both are of the same species, fig. 5 

 having the falcate lobes broken ofll] 



Body segments 11 ; the axis is convex, and very broad, and in 

 front nearly three times the width of the diminutive pleurae ; 

 posteriorly it is narrowed to one third of its width in front, and in 

 the last segment is not quite twice as broad as the pleurae, which 

 diminish but little as they recede from the head. The seventh 

 pleura on each side is broken ; but enough remains to show that it 

 was not in this specimen materially larger than the rest. All are 

 short, falcate, directed backwards and downwards, with a very short 

 oblique furrow, and furnished on the forward edge close to the axis, 

 with a strong projecting fulcral tubercle ; the hinder edge with a 

 corresponding deep notch, the margin of which is raised all round. 

 The pleurae are striated obliquely. The segments of the axis are 

 crossed by tranverse lines, and covered with faint tubercles ; a row 

 of strong tubercles along their hinder edge gives a serrate margin 

 to each segment. There is no appearance of any enlargement of the 

 eighth segment of the axis. Tail squarish, wider in front than 



