126 CARBONICOLA, ANTHRACOMYA, AND NATADITES. 
Family MYTILID AS. 
Genus Natapites, Dawson, 1860. 
G. F. Myles, ‘Memorabilia Saxonie Subterraniz,’ p. 39, pl. xxvi, fig. 10, 1720. 
Synonyms : 

Modiolus, Fleming, 1825. Myalina, Salter, 1861. 
Mytilus, Fleming, 1828. Anthracomya ? Salter, 1861. 
Modiola, Sowerby, 1840.! Anthracoptera, Salter, 1862. 
Mytilus, Sowerby, 1840. Anthracomya, Hull, 1873. 
Avicula, Sowerby, 1840. Myalina, R. Etheridge, jun., 1875. 
[No name], R. Garner, 1844. Anthracoptera, R. Etheridge, jun., 1875. 
Modiola, Brown, 1849. = Ward, 1875. 
Avicula, Brown, 1849. Myalina, Kirkby, 1880. 
Mytilus, de Ryckholt, 1852. Conocardium, Achepol, 1880-83. 
Modiola, Lea, 1853. Dreissenia, Achepol, 1880-83. 
Dreissenia, Ludwig, 1859-61. Mytilus, Achepol, 1880-83. 
Myalina, McChesney, 1860. Myalina, Barrois, 1882. 
— Meek and Worthen, 1860. Anthracoptera, Hind, 1893. 
Naiadites (pars), Dawson, 1860. Naiadites, Hind, 1894. 
Generic Description.—Shell modioliform, obliquely triangular, very unequi- 
lateral, close all round ; inequivalve, the left valve being the more tumid, the right 
valve flattened. Both valves notched for the byssus, the left one being so cut to 
a greater extent. The anterior end is oblique, forming a small lobe anterior to 
the umbonal ridge. There is always present a more or less defined oblique 
ridge passing from the umbones backwards and downwards, most marked 
in the left valve. The hinge-line is straight. The hinge-plate is striated, 
and possesses in front an obscure cardinal tooth, that of the left valve being 
anterior, and that in the right posterior. The umbones are almost terminal, and 
generally pointed forwards. The shell is strong and even massive anteriorly, 
becoming gradually thinner posteriorly, till at the extreme end it may be very 
fragile. The surface is furnished with flat concentric lamelle, and lines of growth. 
Periostracum thick and wrinkled. Anteriorly the lines of growth are close and 
crowded, but they separate as they pass backwards and become distinct ; and when 
they reach the oblique ridge they become reflected rapidly upwards towards the 
hinge-line. I have seen no trace of internal or external ligament, but the place of 
! See foot-note, p. 83. 
