286 ICHNIUM AND OLDHAMIA. [May I9OO, 



Plate XVIII. 



[The figures of this Plate are reproduced from photographs of actual specimens, 

 and are uniformly magnified 44 diameters.] 



Kg. 5. Central region of a system of Oldhamia radiata. Many of the radial 

 grooves are constricted at intervals, so as to form a series of pits. 



6. Part of a system of 0. antiqua, exposed on the surface of a slab of 



indurated shale, for comparison with fig. 8. 



7. Several of the rajs of 0. antiqua, showing a beaded appearance. This 



will be most readily seen by viewing the figure from the left-hand 

 side. 



8. Part of the same system of 0. antiqua as that shown in fig. 6, but 



exposed un a polished surface parallel to that of fig. 6, lying 3 mm. 

 below it. To compare the two figures, fig. 8 should be viewed in a 

 mirror, and the obtuse angle in the outline of the upper right-hand 

 side made to correspond with the similar angle on the lower left-hand 

 side of fig. 6. 



9. View en fare of one of the oval depressions on the surface of the shale, 



often associated with Oldhamia. Owing to its conical form, it cannot 

 be brought uniformly into focus. 



10. The structure, which descends through the rock from such a depression 



as that shown in the previous figure, exposed in horizontal section, 

 3 mm. below the level of the surface on which the depression occurs. 



Plate XIX. 



[The figures of this Plate are reproduced from photographs of actual 

 "specimens.] 



Fig. 11. Central area of a fan of Oldhamia antiqua, showing the smooth oval 

 area, from which the rays originate. X 4 - 4. 



12. Isolated rays of Oldhamia ; the two upper parallel ridges are bordered 

 by faint surrounding mounds. X 4*4. 



13. An isolated spray of 0. radiata, showing three club-shaped terminal 

 processes. X 4*4. 



14. Part of a vertical section through a slab of rock containing Oldhamia. 



A little below the centre, and slightly to the left, a single groove is 

 clearly displayed ; it is a section across a single Oldhamia-vay . An 

 almost perfect circle (black in the photograph) rests in the groove, 

 and a linear disturbance of the laminae of the rock is seen on each 

 side outside the groove running transversely. Indications of a cir- 

 cular arrangement of particles are common throughout the general 

 substance of the rock. X 22. 



15. Vertical section through Oldhamia-be&ring rock. X 4"4. 



16. Part of a similar section, to show scattered concentric systems of 

 circles. X 22. 



Discussion. 



Mr. E. T. Newton and Prof. W. W. Watts spoke. 



The Author thanked the Fellows for the favourable reception 

 which they had given to this communication. The subject was not 

 yet exhausted, and it was possible that with further investigation 

 some of the points that still appeared obscure would be elucidated. 



[Postscript. — The crustacean mentioned on p. 276 is an amphipod, and it 

 has been kindly determined for me by Dr. Goodrich as Corophium lonaicorne, 

 Lat.— W. J. S., April 26th, 1900.] 



