MEASURES OF THE TYNE VALLEY 515 



By making use of the imprint borne by the other half of the 

 nodule it is possible to trace a practically complete outline on 

 the right side, and a restoration based upon this gives the 

 shape shown in fig. 4. The whole, or almost the whole 

 surface of the abdominal plates is finely granulate. This is 

 perhaps inconsistent with the supposition that it is the inner 

 surface ; but it must be noted that the pustules forming the 

 granulation are very small indeed. (In the cephalothorax 

 also there are small areas of similar granulate integument). 

 A strong ridge runs nearly down the centre of the anterior 

 abdominal segments (see figs. 2 and 3) and certainly gives 

 this part of the fossil the appearance of being the dorsal 

 surface. It may, however, be explained in another way. Just 

 as the imprint of ventral structures is so commonly produced 

 on the dorsal plates, the moulding of the dorsal plates may 

 doubtless be imparted to the ventral. Now in Anthracosiro 

 and several other related genera there is a ridge or keel down 

 the centre of the back; in Anthracosiro it is described by 

 Pocock as consisting of a series of median triangular crests, 

 one on each tergite (fig. 5). The ridge present in this speci- 

 men is exactly such as might be produced in such a case in 

 the ventral plates,- when the internal tissues decayed and the 

 pressure brought the dorsal and ventral integuments together. 

 If, as was probably the case, the dorsal integument was the 

 stronger, it would be the dorsal integument that would give 

 its shape to the fossil in so far as the latter was not flattened 

 out altogether under the pressure; the ventral integument 

 would in fact be forced up into the dorsal as into a mould. 

 The same supposition would account for the convexity of the 

 cephalothorax. The ridge ends abruptly in the sixth segment, 

 and behind that point the abdomen presents a shallow, even 

 concavity. The wrinkles about the middle of the right side 

 are no doubt produced by some accident of fossilization ; and 

 there has also been some distortion of the outer margin of 

 the plates, for whereas this margin is somewhat turned up 

 (concave) on the right side, on the left it is bent down sharply 

 into the matrix. Where the margin of the abdominal plates is 



