142 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



tion, retaining the name Avicula, and this probably is the reason why he describes in 

 the same volume a shell (Modiola producta) which from his figure I take to be the 

 same as Sowcrby's form. It is said to occur in coal shale at Wakefield, but no 

 notice is given in the description of any definite shape or characteristics. The 

 posterior end of Captain Brown's specimen was imperfect, so that it is impossible 

 to give a dogmatic opinion as to the real nature of the shell. In my opinion it 

 was either a large form of N. carinata, or more probably a specimen of 

 .V. quadrata. 



With regard to the American specimens, I am judging from figures and 

 descriptions only, for I have been unable to obtain access to forms from Missouri 

 and Illinois. The figures and descriptions, however, show such a close connection 

 with British forms that I have ventured to place Myalina aubquadrata and 

 M. meliniformis as doubtful synonyms of Sowerby's species N. quadrata. 



This form is comparatively rare ; it occurs on an average of only one to a 

 hundred of the other species in the Hard-mine of North Staffordshire. The charac- 

 teristic flattened U-shaped form with a much expanded posterior wing separate it 

 from other species. The oblique ridge, too, is more nearly at a right angle to the 

 hingre-line. It is connected with N. modiolaris on the one side and Naiadites 

 carinata by a series of intermediate examples, forming with the three foregoing 

 species one natural group which appears to be developing into four different 

 distinct forms, characterised mainly by changes in the rate of growth of the shell 

 at different points, the concomitant variations in anatomy depending on this. 



■V Naiadites elongata, llivd, 1883. Plate XVIII, figs. 22, 26—35. 



Naiadites elongata, Mind. Quart. Journ. Geol. Soc, vol. xlix, p. 256, pi. vii, 



figs. 15, 15 a, 10, 17, 1883. 

 Non Dawson. Acadian Geology, 1st edit., p. 43; 2nd edit., 



p. 202, fig. 43. 



Specific Characters. — Shell tumid, modioliform, transversely elongated, only 

 slightly equivalve. The anterior end is short and swollen, the posterior obliquely 

 tumid below, flattened and compressed above so as to become concave between the 

 hinge-line and the oblique gibbose keel, which pusses downwards and backwards 

 from the umbones to the posterior inferior angle, preserving its form, though 

 somewhat expanded and less elevated to the i^\^ 1 of the shell. The anterior end. 

 has a blunt convex border, continuous with the inferior border, which is straight 

 and directed downwards and backwards, having the byssal notch about the middle 



