350 Proceedings of the Royal Physical Society. 



It would be specially interesting to know what forms lived 

 between the Neolimulus of Upper Silurian and the Prest- 

 witchia and Bellinurus of Carboniferous times. 



Explanation of Plate. 



Fig. 1. Pterygotus Dicki, n. spec. Antepenultimate segment, reduced 

 3 diameters, showing exterior of ventral surface, the dorsal surface, partly 

 as a cast, and portion of its interior, also portion of posterior flange. Fig. la. 

 Portion of the under surface of flange and marginal fringe, to show nature of 

 ornamentation, — natural size. Fig. lb. Portion of upper surface of marginal 

 fringe, — natural size ; from Lower Old Red Sandstone, Thurso, Caithness ; 

 now in the collection of the Museum of Science and Art, Edinburgh. 

 Fig. Ic. Diagrammatic restoration of the antepenultimate segment of Ptery- 

 gotus Dicki. 



Fig. 2. Diagram to show the manner in which the angle of the sloping 

 sides of Fig. 1 has been lessened by compression of the matrix. The dotted 

 lines show the position before, and the firm lines the position after, compres- 

 sion. Fig. 2a. Diagram showing the manner of compression of the vertical 

 sides of the same ring. The dotted and firm lines bear the same relation to 

 each other as in Fig. 2. 



Fig. 3. Diagram to show the formation of "Adam's Plate" from tail ring 

 of Pterygotus. a, Dotted lines, showing ring before compression ; b, firm 

 lines, ring after compression ; c, section of flag, with that portion of ring 

 concerned in the formation of an "Adam's Plate," in its natural position. 

 Fig. 3c?. Section of above flagstone turned upside down. 



Fig. 4. Outline of cast of Limuloid allied to Bellinurus ; from the Lower 

 Old Red Sandstone, near John o' Groat's House, Caithness. From C. W. 

 Peach's Collection. 



