NATADITES. 127 
a ligament was taken by the fibro-cartilage which was attached to the striations on 
the hinge-plate. 
Interior.—The pallial line is represented by a series of small pits and is not 
sinuated. The posterior adductor scar is almost circular, large, and situated near 
the posterior border. The anterior adductor scar is small, pit-like, and is situated 
within the beak somewhat anterior to the umbo. There are two accessory pit-like 
scars for the pedal and anterior byssal muscle; one is umbonal, and the other 
is midway between the umbo and the anterior adductor scar. The scars for 
the byssal muscles are almost obsolete. 
Observations.—In the very first work that I can find on the subject of fossil 
bivalves from the Coal-measures, by G. F. Myles, dated 1720 (op. supra cit.), 
figures are given of the genus Naiadites, but unfortunately no name was given to 
them (vide antea, p. 12). Since that time the genus has passed through many 
changes in nomenclature, and it has been a matter of some little difficulty to 
decide as to the priority of the different claims. Except in one case, authors have 
always referred the shells of this genus to one of two families, the Aviculide 
and the Mytilide; the exception being in the case of Achepol, who referred 
specimens to Conocardium, a genus which has really no characters in common 
with Naiadites. The reference to Avicula was made only by two authors ; one of 
whom was J. de Carle Sowerby, and the other Captain Brown, who simply 
repeated Sowerby’s figure and description. 
Sowerby curiously enough described four species of this genus under three 
different generic names in the same publication (‘ Geol. Coalbrookdale’) ; namely, 
Modiola, Mytilus, and two under Avicula, all four figures being good and 
characteristic. The genus Naiadites can easily be separated from Avicula, as it 
possesses no ears nor cartilage-pit, and the valves, though unequal, are not greatly 
so. The general shape, too, is very dissimilar. 
To Modiola and Mytilus the genus Naiadites seems to be very closely related, 
but it has certain distinctive characters which separate it from both of them. 
From Modiola, Naiadites is distinguished by the possession of a striated hinge- 
plate, the absence of an internal ligament, and by being less transversely pro- 
duced. From Mytilus Naiadites is separated by the non-terminal umbones, the 
absence of internal ligament, and the striated hinge-plate. The possession of 
this latter character points to the fact that Naiadites is intermediate between the 
Aviculide and the Mytilide. 
Ludwig referred his specimens of Naiadites to Dreissenia, to which genus there 
is a very close external resemblance, both in shape and ornament; but Dreissenia 
possesses an internal septum in the beaks, which I have never seen to occur in 
Naiadites; and in Dreissenia the posterior adductor muscle is riband-hke and 
