212 PROCEEDINGS OP THE MALACOLOGICAL SOCIETT. 



without any possibility of these facts being attributable to retro- 

 gression." ^ 



Further, he observes that the Mytilidae are remarkable for the 

 important part taken by the pro vinculum, and for the manner in 

 which the permanent ligament is developed. At the close of the 

 embryonic stage there is in this, as in all the allied families, a sudden 

 change in the growth of the shell, a dissoconch being formed, which is 

 gradually enlarged into the adult shell. In this process Jackson has 

 recognised two transitory stages, which he has called the nepionic and 

 the nealogic. In the Mytilidae that which becomes the permanent 

 ligament is developed separately in the nepionic stage, a second small 

 ligamental pit appearing on the hinge-line or provinculum, behind the 

 central ligament-pit of the prodissoconch. 



From the time when this second ligamental pit makes its appearance, 

 the creuulations on the posterior part of the provinculum are greatly 

 modified ; those which lie between the two ligamental pits become 

 pinched up, while those behind the second pit increase in number, 

 and the ligament extends itself posteriorly below the creuulations as 

 the provinculum grows and extends in that direction. 



With the commencement of the succeeding nealogic stage, the 

 growth of the crenulated band or plate is stopped, and teeth of 

 another kind begin to form at each end of it. These are traceable to 

 the primitive riblets of the shell, and for them Bernard adopts 

 Neumayer's name of dysodont teeth. The hinge-line is thus continued 

 and extended by a series of dysodont teeth, and, at the same time, 

 the posterior ligament increases in length until it passes beyond the 

 limit of the provinculum and reaches the posterior dysodont teeth, 

 sometimes remaining below these teeth, as in Crenella, but generally 

 spreading upward, and erasing all the provincular teeth and sometimes 

 all of the dysodont teeth as well. 



On the anterior side of the shell other changes are in progress 

 during this nealogic stage. This side is bent gradually inward, or, as 

 Bernard expresses it, rotated on an axis, so that it encroaches more 

 and more on the anterior part of the primitive crenulated hinge-plate. 

 This process is continued till, in 3Iytilm, all the anterior part of the 

 original crenulated band is obliterated by the incurved border with its 

 dysodont teeth, the earlier dysodont teeth being thus brought beneath 

 the umbo, while later teeth appear on the border below and in 

 front of it. 



The final result in a typical Mytilus is that the primitive central 

 ligament-pit is obliterated by the incurvature of the anterior border, 

 and the posterior teeth are generally obliterated by the growth and 

 extension of the permanent posterior ligament, so that in most cases 

 the hinge consists entirely of the anterior dysodont teeth, a few of 

 which generally persist beneath the umbones of the adult. Some- 

 times, however, even these disappear as separate teeth, and the hinge 

 becomes almost edentulous. 



1 Ann. des Sci. Nat., Zool., torn, viii, p. 67 (1898). 



