JUKES- BROWNE : ON THE MYTILIDiE. 213 



There ai'e, however, many exceptions to this obliteration of the 

 posterior denticulations ; in one group of shells (hitherto included in 

 the genus Mytilus) all these posterior teeth persist in the adult, the 

 ligament I'emaining entirely internal ; in others many of the dysodont 

 teeth are effaced by the upward growth of the ligament, but some still 

 remain beyond its termination. 



M. exustus (Linn.) is a species in which all the posterior teeth thus 

 persist. The hinge of a young specimen of this species was figured 

 by Bernard, but he does not seem to have been aware that the stage 

 which he figures persists in the adult. He makes it clear, however, 

 that these posterior crenulations are of exactly the same nature as the 

 anterior denticles, the position of the latter below the hinge and umbo 

 being due entirely to the incurvature of the shell during the disso- 

 conch stage. 



It would appear, therefore, that Bernard's work makes it necessary 

 for conchologists to reconsider the generic values of the characters 

 presented by the shells of the Mytilidae, and to revise the genera and 

 subgenera, so as to establish these groups on a sounder and more 

 satisfactory basis. 



In the first place it seems to me that those species of Mytilus which 

 possess posterior denticulations on the hinge-line of the adult shell 

 should be separated from the species which do not present this 

 character, and should be grouped in a distinct genus. There is 

 a better reason for such separation than at present exists for separating 

 Mt/tilus from Modiola^ for a typical Modiola differs from an edentulous 

 Mytilus only by a less amount of rotation and incurvature of its 

 anterior side, whereas a denticulate Mytilus retains an important 

 embryonic character of the hinge-line ; and it is generally admitted 

 that a hinge-character is of more importance than one which merely 

 affects the shape of the shell. 



Again, there are some species, at present included in the genus 

 Modiola, which possess a similar row of posterior denticulations. Of 

 this group Modiola citrina (Chemn.) is the type, and if this be 

 compared with Mytilus exustus and M. Dominyettsis it will, I think, 

 be apparent to everyone that there is a much closer resemblance 

 between all three than there is between Modiola citrina and Modiola 

 modiolus. There are many species of Mytilus in which the umbones 

 are by no means terminal, and some of these have sometimes been 

 referred to Mytilus and sometimes to Modiola. 



The union of the two groups typified by M. citrinus and M. Doming- 

 ensis has been advocated by Dr. H. von Ihering, and on this point 

 I quite agree with him, only I lay more stress on the denticulation of 

 the hinge-line than he does, and less on the surface-structure of the 

 shell. I would therefore raise the assemblage to the rank of a genus, 

 and should include in it the group for which the name Mytilaster was 

 proposed by the Marquis de Monterosato in 1884. 



In this connection I would remark that the sculpture of the 

 external surface of the shell does not seem to be a character of mucli 

 importance in the Mytilidae. If it were always correlated with other 

 differential characters it might be so regarded, but this is not the 



