0. 0. Robson — On Helminthochiton. 



303 



sculpture is seen exteuding as two or three faint grooves from the 

 carina at one end to the free angle at the other (emarginate) end. 

 Under a moderate power of the microscope, but scarcely visible with 

 a hand lens, a faint concentric striation, like growth-lines, is visible 

 running parallel to the outer margins. The valves themselves are 

 very small, a typical example measuring 5 mm. along the carina, and 

 3'5 mm. from the carina to the inferior free border. 



It is impossible to distinguish any other characters than these. 

 Thus it will be seen that the data available for determining the 

 nature and affinities of this fossil are very scanty. The bent plates 

 with diagonal and concentric sculpture suggest a Chiton more than 

 any other form, and we shall see that one character at least — the 

 (posterior) emargination — suggests a definite genus of Chiton. One 

 or two of the plates look superficially like the carinals of an unorna- 

 mented Cirripede, though there is nothing beyond the carination to 

 endorse this view, and Messrs. Cowper Reed and Withers have not 

 upheld it after examining the specimens. 



Ant. 



Post. 





1. 3. 



Helminthochiton ceqiiivoca. x 6. Lateral view of plates, a — a, median line ; 

 6, the posterior emargination. 



Reference to the literature of Ordovician and Silurian inverte- 

 brata has yielded only one genus to which this fossil might be 

 referred, viz. Salter's Helminthochiton (1846-7) as redefined by 

 de Rochebrune (1883). Through the kindness of Mrs. Robert Gray 

 and Dr. R. F. Scharff it has been possible to make a careful study 

 of S. grayicB, Woodward (1885), from the Starfish Bed of the 

 Drumrauck group of Girvan (Ashgillian age), and of S. grifithii, 

 Salter, from the Silurian (Upper Llandovery) mudstones of Galway. 

 The type of the latter from the National Museum, Dublin, was lent 

 by Dr. Scharjff, while ample material representative of the former 

 was lent by Mrs. Gray. 



The Bohemian fossil seems to be adequately distinguished from 

 the Girvan form. The only feature in common between them is 



