136 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
? MYALINA TRIANGULARIS, Barrois. Recherches sur les terrains anciens des 
Asturies et de la Gallice, p. 336, pl. xvi, 
fig. 14, 1882. 
ANTHRACOPTERA TRIANGULARIS, Hind. Quart. Journ. Geol. Soe., vol. ]xix, p. 253, 
pl. vii, figs. 7—9 a, 1893. 
Specific Characters.—Shell mytiliform, compressed obliquely, obovate with a 
pointed anterior end, and the posterior wall, much flattened and produced obliquely 
downwards and backwards, comprising the greater part of the shell. The 
posterior border is regularly curved almost in a semicircle, and it passes imper- 
ceptibly into the upper and lower borders. The hinge-line is straight, equal in 
length to about one-third of the greatest length of the shell. At the posterior end 
of the hinge-line the superior border of the shell slopes downwards and backwards, 
passing gradually into the posterior border with a gentle curve. The inferior 
border is very oblique and nearly straight. The byssal notch is situated at the 
junction of the anterior and middle thirds. The umbones are not so tumid as in the 
preceding species, but similar in shape and direction. An oblique ridge passes 
obliquely downwards and backwards, but soon becomes lost on the surface of the 
shell. 
Interior and external markings as in Naiadites modiolaris. 
Dimensions.—Fig. 31, Pl. XVII, measures— 
Greatest antero-posterior . 38 mm. 
Greatest dorso-ventral (close to nontouen end) . 21 mm. 
Thickness. . 10mm. 
Localities.—England Sonne of the Hand: -mine Coal, North Staffordshire. 
Grains Ironstone, South Staffordshire. Crawstone, Coalbrookdale. Blue Vein, 
Ebbw Vale. Scotland :—Shotts. 
Observations.—This shell only differs from Naiadites modiolaris in shape, and 
is distinguished by its flatter form and the shape of the posterior end; a greater 
amount of growth having taken place towards the posterior inferior extremity, 
with the result of making the hinge-line much shorter than the extreme length of 
the shell. It is very doubtful whether this condition should be regarded as of 
specific value, but I have retained the name for the present because it is quite 
easy to distinguish between the two forms, and typical young examples occur 
along with the adults. At the same time it is true that many specimens occur 
which it is difficult to determine, and a series might easily be figured connecting 
the forms N. modiolaris and N. triangularis on the one hand, and N. triangularis 
and N. carinata on the other. Sowerby’s original figure was from an imperfect 
specimen, which I am unable to trace. This was incomplete at the posterior 
superior angle, and a hypothetical outline was dotted in. This specimen must 
have been juvenile, for its extreme antero-posterior measurement is only 18 mm., 
