NAIADITES ELONGATA. 143 



point, the notch being chiefly in the right valve. The posterior border forms an 

 obtuse angle above with the hinge-line, from which point it slopes downwards and 

 backwards, becoming bluntly truncate below, or even subangular, where it comes 

 in contact with the inferior border. 



The hinge-line is straight, equal to about two thirds of the length of the shell, 

 and would, if produced forwards, make a very acute angle with a line drawn 

 in the direction of the inferior border. The umbones are obtuse, not quite 

 terminal, produced forwards, non-contiguous, continuous below with the oblique 

 ridge. The byssal furrow anterior to the ridge is oblique, broad, and much more 

 marked in the right valve than in the left. 



Interior. — The same conditions obtain in this form which I have noted in the 

 preceding species, but the anterior muscle-pits are closer together and smaller on 

 account of the size of the shell. 



Exterior. — The lines of growth have the same general arrangement which exists 

 in the other species of this genus, modified of course according to the difference 

 of shape which is characteristic of the present shell, but on the whole the lines 

 of growth are closer and more numerous than in other species. 



Dimensions (Fig. 31, PI. XVIII) : 



Antero-posteriorly . . . .26 mm. 



Dorso-ventrally . . . .13 mm. 



From side to side . . . .10 mm. 



. Localities. — The Knowles or Winghay Ironstone of North Staffordshire. 



Observations. — This is an interesting species, making as it does by its external 

 appearance a link between Naiadites and Anthracomya, Naiadites elongata 

 very closely resembling Anlliractnuija ml ulnar. The resemblance is so close 

 that it is impossible to separate imperfect examples ; and it has occurred to 

 me that this may be an example of protective mimicry rather than an actual 

 closer connection between the two genera. The examples of N. elongata which 

 have occurred to me nearly all possess the shell well preserved, and therefore 

 the characteristic anterior triple muscular scars are not available for generic 

 determination, but I have satisfied myself that they are present. 



The umbones, too, have the distinctive appearance which obtains in Naiadites ; 

 but, being small and inconspicuous, are often not well freed from their matrix, 

 which as far as I can find at present is always ironstone, and on this 

 account very difficult to remove. Another distinctive feature which may be 

 relied upon to distinguish Naiadites from Anthracomya is the inequality of 

 the valves in the former. This is always present, though less marked in 

 this species than in others. The absence or presence of a byssal notch is 

 another important distinction, being always present in Naiadites. From a study 

 of a series I have come to the conclusion that when young the shape of the shell 



