NAIADITES TRIANGULARIS. 137 
and the dorso-ventral measurement is only 8 mm., so that the shell is long 
compared to its depth. 
The description given is meagre, being “ Triangular, with a rounded front, rather 
flattened, keeled towards the beaks.” It is therefore a question as to which 
adult forms should be referred to under this name. Judging from shape alone, 
N. modiolaris merits the term triangular, but the much greater comparative 
dorso-ventral measurements, and the fact that Sowerby’s original figure of 
Avicula modiolaris is very typical of that form of the shell which possesses a hinge- 
line somewhat shorter than the length of the shell, make it impossible to consider 
that the more triangular forms of this shell should be referred to N. triangularis. 
Again, emphasis is laid upon the fact that the shell is rather flat, and the drawing 
agrees with this description, and this prevented me from referring the new species, 
what I named Naiadites elongata in 1883 (op. infra cit.), to Sowerby’s shell. In 
N. elongata the hinge-line is long and the shell transverse, but it is very much 
swollen in its entire length (Pl. XVIII, fig. 31). 
In young and medium-sized specimens of N. triangularis (Pl. XVII, fig. 38) it is 
comparatively easy to imagine that the whole of the superior border is hinge-line, 
as, owing to the very gradual slope of the posterior part of the shell, the line of 
the hinge and posterior edge of the shell appear to be continuous; but on 
careful inspection this is seen not to be so. The hinge-line is comparatively short 
and the posterior border begins to fall away at once. This is shown in Sowerby’s 
5) 
drawing, ‘Trans. Geol. Soc.,’ ser. 2, vol. v, pl. xxxix, fig. 18, left-hand figure, 
and the dotted line is carried forwards past the point to the posterior extremity 
of the hinge, and a fictitious posterior end, in its upper part, depicted. In young 
shells the posterior end is more truncate and less rounded (Pl. XVI, figs. 36 
and 38), and the anterior end more lobe-lhke and pronounced. 
Myalina Swallovi of McChesney, Meek and Worthen, is, I think, doubtless 
referable to N. triangularis. They figure a full-grown form not to be distinguished 
from the specimen I figure (Pl. XVII, fig. 36). 
I think it probable that M. Toilliezianus and M. ampelitecola of de Ryckholt 
should all be referred to N. triangularis, the two names being given to young 
forms having the anterior end lobe-like, as in Pl. XVII, fig. 836. The former is 
stated to have been obtained from the coal shales of Mons, the latter from 
99 
“Pampélite alumineux”’ near Liége. 
The figure given by M. Barrois is not lke the original; it is subquadrate 
in shape, and in no way transverse, resembling rather a young specimen of 
N. quadrata. 
Naiadites triangularis is to be distinguished by its obliquely triangular, 
flattened form, short hinge-line, and gradually curved posterior end, and by the 
oblique ridge extending only a short distance across the shell. 
18 
