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. 
Intevior.—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, Pl. XVIII): 

Antero-posteriorly é : , . 26 mm. 
Dorso-ventrally : : A ba) ls) mamas 
From side to side : : » 10mm: 
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, Natadites elongata 
very closely resembling Anthracomya minima. 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 
