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SUPPLEMENT TO THE BRITISH 



When I published my description of T. ovoides, Sow., in June, 1851, I had arrived 

 at the correct conclusion that Tereb. ovoides and T. lata, Sow., from the Drift of 

 Norfolk, were referable to a single species, but I committed a great mistake in placing 

 Young and Bird's Tereb. trilineata, from the Inferior Oolite of Robin's Hood Bay, among 

 the synonyms of Sowerby's shell. There exists a certain resemblance between some 

 specimens of the two species, but it is possible to distinguish them by several 

 features. 



The discovery of my mistake is due to Mr. Lankester and Mr. Walker, and I 

 feel much indebted to those two gentlemen for having put me on the right track. 



The boulders in which Ter. ovoides occurs are, as stated by Mr. James Sovverby, "a 

 sandstone containing green sand," and very different in colour and composition from the 

 rock which, at Peak and Glaizedale, near Whitby, contains the Ter. trilineata of Young 

 and Bird. 



Sowerby's figure of T. ovoides is not very good, and the three-quarter view in which 

 the shell is drawn is an unfavorable position. 



Terebratula ovoides is longer than wide ; and its greatest breadth is at the posterior 

 half of the shell, whence the sides gradually taper to the small rounded front. 

 In Ter. trilineata the shell is comparatively less elongated, more irregularly oval or 

 ovate. Again, in T. ovoides the ventral valve is very convex and longitudinally keeled ; 

 while in T. trilineata the same valve is more regularly rounded. The dorsal valve is 

 also deeper and more uniformly convex in the last-named shell, the same valve being 

 much more flattened and slightly depressed along the middle in the Sowerbian species. 

 In well-preserved examples of T. ovoides we likewise observe indications of slightly marked 

 radiating lines, which are intersected at various intervals by more or less deeply marked 

 concentric lines of growth. 1 



There exists also a considerable difference in the respective proportions attained by 

 the two species. Thus, for example, the largest specimen of T ovoides that has fallen under 

 my notice measured 3f inches in length by 2f inches in breadth and If inch in depth ; 

 whilst the largest T. trilineata did not much exceed, as far as I am aware, 2 inches in 

 length by 1-f inch in breadth and If inch in depth. 



Terebratula ovoides cannot be confounded with Tereb. homolo(/asti/r, from the ' Brown 



1 In his paper "On a new large Terebratula occurring in East Anglia," published in the 1 Geol. Mag.,' 

 vol. vii, p. 410, Sept., 18/0, Mr. E. R. Lankester expresses the opinion that the large Terebratula drawn 

 as fig. 1 on his plate, and reproduced in our PI. I, fig. 13, has the general simple form of T. ovoides, 

 but is remarkable for its great size ; that the imperforate valve is flattened on the mesial line, 

 whilst the perforate valve is deep and raised into a well-pronounced keel in the mesial line, extending from 

 the beak ; the foramen being likewise small. To this form he has given the name of Rex, and although 

 I will not positively assert that it may not be specifically distinct from T. ovoides, my impression is that 

 it is only a very large example of Sowerby's species. 



