BRITISH FOSSILS. 69 



linear piece, somewhat dilated at the base, convex along the median 

 line, deflexed at the sides, and expanding at the base into a 

 broad arrow-headed plate, the point acute, subtending a variable 

 angle (generally about sixty degrees), and reaching nearly to the 

 front. This portion is divided from the general surface by sutures, 

 but not quite separable. The apex of the central lobe is expanded, 

 rounded, and marked with radiating plaits. As in all other parts 

 of the crust the plicse which closely cover the surface are transverse 

 and but little curved in front, but become semicircular further back, 

 and then semi-ellipses, and even cones, closely tiling over one 

 another. It is this feather-like arrangement, combined with the 

 shape, which must have suggested the term " seraphim." The ex- 

 treme margin is smooth. The plicaB on the arrow-headed central 

 piece of the labrum follow the same order from before backwards, 

 and become elongate and prominent along the central ridge, forming 

 tesselloe rather than scales. 



Body Joints, Plates IV. and V. The segments of the thorax and 

 abdomen may easily be arranged in their right order, from com- 

 parison with those of the preceding more perfect species. Plate IV. 



FIG. 9. 



Pterygotv.8 ang- 

 licus, from the 

 second to the 

 ninth body ring; 

 one-fifth the na- 

 tural size. 



Lowest beds of 

 Old Red Sand- 

 stone, Tealing, 

 Dundee. This 

 specimen is in 

 the Museum of 

 the Watt Insti- 

 tution, Dundee. 



\. 



