NAIADITES. 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 antra, 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 Aviculidaa 

 and the Mytilida3 ; 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 

 Aviculidas and the Mytilidas. 



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-like and 



