MYALINA. 107 



Stoliczka includes the genus Myalina in bis scheme of the Lamellibranchiata 

 (op. cit.), classifying it with Hoplomytilus, Sandberger, Anthracoptera, Salter 

 (= Naiadites, Dawson), Septifer, Recluz, Dreissena (Dreissensia), Van Beneden, 

 and Congeria, Partsch, under the sub-family Dreisseninse, to which Mytilarca, Hall, 

 Plethomytilus, Hall, and Gosseletia, Barrois, belong. All these genera have many 

 points in common, and I have retained this division of the family Mytilidse. 



The shells included in this sub-family all show a very close external resemblance. 

 Myalina possesses terminal umbones, septiferous beaks, and a striated hinge- 

 plate ; Naiadites a striated hinge-plate, but non-terminal umbones, and no rostral 

 plates ; Septifer and Dreissensia, septiferous beaks and terminal umbones, but not 

 a striated hinge-plate ; Hoplomytilus has septiferous terminal umbones, but, 

 instead of a striated hinge-plate, long, linear, transverse teeth ; Congeria is given 

 by Woodward as a synonym of Dreissensia, but Fischer retains it as a sub-genus 

 of that group ; it has a thickened cardinal plate and a rostral septum, but the 

 mantle-line is almost marginal. 



Myalina and Hoplomytilus occur in Devonian rocks ; I am not aware of any 

 named specimens from older strata, but Mr. Whidborne states of the range of 

 Myalina that it ''appears to extend from the Silurian to the Trias." 



Myalina would seem to be at one end or other of the group included under 

 Stoliczka's family Dreisseninx, possessing the three characters, of which one or 

 two only are found to obtain in the other genera. It may have been a parent- or 

 race-form from which the others were developed by the suppression of one or 

 other character. It would appear to have had the longest existence in point of 

 time, and to have been richer in species than any of the other Palaeozoic genera of 

 the sub-family, though Naiadites (with eight British and possibly three American 

 species) is not far behind in point of numbers. Septifer is, I believe, at present 

 only known in the recent state ; but Dreissensia is stated by Woodward to 

 contain fifteen species, distributed in Europe, America, and Africa, and thirteen 

 fossil (Eocene) forms. 



The sub-family Dresseninse forms a distinct link between the Aviculidse and the 

 Mytilidse, both in form and anatomy. The striated, expanded hinge-plate of 

 Myalina, however, is possessed by other genera of shells belonging to the same 

 family, and is not in any way generic in character ; and while, on one hand, these 

 shells have in this hinge-plate a feature connecting them with the Aviculidse, on 

 the other they evidently are related to the Arcidas, which possess, in addition to 

 many hinge-teeth, a well-marked striated ligamental area. Parallelodon (Modiola) 

 squamifer, Phillips, figured PL VIII, figs. 4 — 8, has a hinge-plate which resembles 

 that of Myalina and its congeners very closely, but it has a typically arciform 

 shell. The long lamelliform posterior teeth of the other species of this genus 

 (Parallelodon), adopted by de Koninck, also recall such a striated hinge-plate. 



