POSIDONOjMYA BECHERI. 27 



straight; umbones small, pointed, placed anterior to the centre of the hinge-line. 

 Shell obliquely but very moderately convex. 



Interior. — Unknown . 



Exterior. — Surface ornamented with numerous concentric grooves and ridges, 

 occasionally decussated by radiating lines. Periostracum well developed. 



Observations. — Bronn originally proposed the name Posidonia for this genus, 

 but, finding that this name had been given to a genus of plants, subsequently 

 changed it to Posidonomya, the type of the genus being Posidonomya Becherl. 

 Portlock referred to this genus a very small shell, which is probably a crustacean. 

 M'Coy described two new species, one of which is retained ; the other, P. costafx, 

 seems to me to be referable to PosidonlcJla minor, Bronn. I, hoAvever, refigure 

 the type now preserved in the Museum of Science and Art, Dublin, PI. VI, fig. 10. 



De Koninck separated the shell Posidoniella vetusta, which had generally been 

 referred to the genus Posidonomija, on very good grounds, which have been noted, 

 p. 86, Vol. I of this monograph. 



It is to be regretted that details of the hinge-plate and interior have not yet 

 been noted. 



M'Coy in 1844 (and again in 1853, ' Brit. Pal. Foss.,' p. 521), following Deshayes, 

 thought that the genus Posidonomi/a might be the " internal plate of an animal 

 allied to Ajjlysin," and referred the genus to the Gasteropoda. In the later 

 work he gives his reasons for such a view at length. I have, however, not 

 been able to accept this view, and retain the genus in the Lamellibranchiata, 

 the presence of a periostracum and the occasional preservation of both valves 

 affording sufficient evidence of the real affinities of the shell. 



PosTDONo:\JYA Becheei, Broiiii, 1837. Plate VI, figs. 11—15. 



Posidonia Becheri, Bronn, 1828. Zeitscb. fiir Min., vol. i, p. 262. 

 Posidonomya Becheki, Bron7i, 1835-7. Leth. Geogn., p. 88, taf. 2, fig. 18. 

 Posidonia Becheki, Goldfiiss, 1863. Petref act. German., pt. ii, p. 112,pl. cxiii, 



fig. 6. 



— — Sowerby, J. de C, 1840. Trans. Geol. Soc, 2n(l scr., vol. v, 



pt. 3, pi. lii, figs. 2—4. 



— TUBEKCULATA, Sowevhy, J. de C, 1840. Ibid., pi. lii, fig. 5. 



— LATERALIS, Sowerbij, J. de C, 1840. Ibid., pi. lii, fig. 1. 



— Becheei, Phillips, 1841. Pal. Foss. Cornwall, etc., p. 4-5, pi. x.\, 



fig. 73. 



— TUBERCULATA, Phillips, 1841. Ibid., p. 44, pi. xx, fig. 72. 



— LATERALIS, Phillips, 1841. Ibid., p. 45, pi. xx, fig. 74. 

 Posidonomya Bechebi, Morris, 1843. Cat. Brit. Foss., 1st edit., p. 117. 



— lateralis, Morris, 1843. Ibid., p. 118. 



