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 (pp. infra cit.), to Sowerby's shell. In 

 JV. elongata the hinge-line is long and the shell transverse, but it is very much 

 swollen in its entire length (PI. XVIII, fig. 31). 



In young and medium-sized specimens of N. triangularis (PI. 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 

 drawing, ' Trans. Geol. Soc.,' ser. 2, vol. v, pi. 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 (PI. XVII, figs. 36 

 and 38), and the anterior end more lobe-like and pronounced. 



Myalina Swallovi of McOhesney, Meek and Worthen, is, I think, doubtless 

 referable to V. triangularis. They figure a full-grown form not to be distinguished 

 from the specimen I figure (PI. XVII, fig. 36). 



I think it probable that M. Toilliezianus and M. ampelitsecola of de Ryckholt 

 should all be referred to V. triangularis, the two names being given to young- 

 forms having the anterior end lobe-like, as in PI. XVII, fig. 36. The former is 

 stated to have been obtained from the coal shales of Mons, the latter from 

 "l'ampelite alumineux " near Liege. 



The figure given by M. Barrois is not like 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 



