Dr. J. E. Gray on some Families of Bivalve Shells. 413 



or rudimentary. Cartilage linear, external. Periostraca hard, 

 polished. Living attached to floating seaweeds. 



1. Modiolarca. Modiola trapezium, Lamk. This is the 

 animal I described as Crenellida, Syn. B. M. 1841, 177; it is 

 very distinct from Crenella or Modiola. 



2. ?Mytihmera. 



Fig. 2. 



Yoldia australis. 



The figure, from a drawing by Albany Hancock, Esq., represents the 

 animal as seen through its transparent mantle, on the removal of the right 

 valve. 



a, a, adductor muscles ; p, p, pedal muscles ; x, x, lateral muscles of the 

 foot ; /, foot ; t, t, labial ]3alpi and appendages ; g, gills ; s, siphons ; 

 m, pallial line ; i, a convolution of the aliraentarj- canal, lying close to the 

 right side, and producing an impression in the shell ; I, hgament. 



IV. Foot elongate, slender, strap-like, byssiferons (for anchoring 



the animal). 



18. Dreissenida, Syn. B. M. 1842,82; P.Z.S. 1847, 199. Foot 

 conical, small ; anterior adductor muscle small, on a transverse 

 subumbonal internal plate. Mantle edge double. Valves sub- 

 trigonal, keeled. Hinge toothless. Cartilage external, marginal. 

 Fluviatile. 



19. Galeommid(B, Syn. B. M. 1842, 78; P. Z. S. 1847, 192. 

 Foot very small, ligulate, flattened beneath. Mantle edge double, 

 outer more or Jess expanded and reflexed, covering (perhaps all) 

 the shell, inner covering the gape of the shell. Gills two pair, 

 dependent, united together behind the foot ; lips elongate. 

 Shell oblong, gaping beneath. Hinge toothless. CartUage 

 internal, in a triangular pit. 



V. Foot very small, rudimentary, byssiferous ; hinder adductor 



muscle large, placed forward in the centre of the lower edge 

 of the shell. Pedal opening small, in front near umbo ; anal 

 opening apparently behind and above the adductor muscle. 



20. Tridacnida, Syn. B. M. 1842, 82; P. Z. S. 1847, 198. 



