211 



A EEVIEW OF THE GENERA OF THE FAMILY MYTILIDJE. 

 By A. J. Jukes-Browne, B.A., F.Gr.S. 



Read \2)th January, 1905. 



The shells of the genus Mijtilus have not been favourites among 

 conchologists, because few of them display any elegance of shape or 

 any beauty of colour. Their dentition has been dismissed as variable, 

 the so-called cardinal teeth being described as small or obsolete, and 

 the crenulations, which often occur on the anterior border and some- 

 times on the dorsal border, have not been regarded as true teeth. 



The recent researches of M. Felix Bernard ' into the development of 

 their hinge-teeth have, however, invested the Mytilidae with a much 

 greater interest for all those who concern themselves with the phylo- 

 genetic classification of the Pelecypoda and with their palseontological 

 history. 



The observations of Neumayer and Bernard have shown that in 

 some genera of Lamellibranchs the embryonic shell (prodissoconch) 

 does not show any crenulations along the hinge-line (e.g. Cytherea, 

 Liicina, etc.) ; while in others this hinge-line is slightly thickened 

 and crenulated, forming a slender and primitive hinge-plate, which 

 Dr. Dall has called the provinculum. In the centre of this pro vinculum 

 there is a small ligamental pit separating the crenulated band into 

 two series, an anterior and a posterior. 



The crenulations of the provinculum seldom persist into the adult 

 stage, other teeth being subsequently developed which take their 

 place, and a more solid hinge-plate being formed in connection with 

 these later teeth. These secondary teeth are known as dysodont 

 teeth in the Mytilidse and as taxodont teeth in the Arcidse and their 

 allies, but Bernard has shown that both kinds have the same origin, 

 and that they spring from the terminations of internal ribs. 



Bernard remarks that the study of the hinge-teeth should commence 

 with the Mytilidae, because the genera of this family "furnish the 

 key of all the problems which present themselves in this period of 

 their evolution. It is, in fact, in this family that the development 

 [of the hinge] is slowest, and consequently the plainest, the stages 

 being clearly separated from one another ; in it, also, the dental 

 products exhibit a primitive character and a minimum of differentiation, 



^ "Notes sur le developpement et la morphologie de la coquille chez les Laraelli- 

 branches " : Bull. Soc. Geol. Fr., 1895 and 1896, vols, xxiii, xxiv. " Sur 

 le developpement des dents de la cliarniere chez les Lamellibranches " : C.R. 

 Acad. Sci. Paris, 1897, vol. cxsiv, p. 1165. " Recherches sur la coquille des 

 Lamellibranches": Ann. des Sci. Nat., 1898, vol. viii, pp. 1-208, pis. i-xii. 



