54 Descriptions of Preparatio7is. 



For the nervous sj^stem of the Cephalous Mollusca^ see Hancock 

 and Embleton, Phil. Trans. 1852, p. 238; Huxley, Phil. 

 Trans. 1853, p. 53- 



For figures of the nerve system of this and other Pulmonate Gras- 

 teropods, see Keferstein, Die Klassen und Ordnungen des 

 Thierreichs, iii. 2, Taf. xcvi., and Walter, Microscopische 

 Studien, 1863, cif. in loc. 



For a figure of the nerve collar with the otic vesicles and aorta in 

 situ, see Leidy in Binney's Terrestrial Molluscs of the United 

 States, vol. i. pi. xvii. fig. iv., and vignette, p. 248. 



For the distribution of the nerves, see Cuvier Ann. du Museum, 

 torn. vii. p. 172, 1806; or Memoires pour servir a FHistoire 

 et a 1' Anatomic des Mollusques. 



18. Shell of Fresh-water Mussel [Anodonta Cygnea). 



When the bivalve shell of the fresh-water mussel, or of any of 

 the British fresh-water species of Lamellibranehiata, is held with 

 its hinge-line upwards, its line of aperture downwards, and its 

 ligament and its umbones, the most convex portions of the valves, 

 placed, the one proximally, and the other distally to the observer, 

 he will then have the animaFs right valve at his right hand, 

 its left valve at his left hand, the anterior portion of its body 

 placed distally, and the posterior proximally to himself. Such 

 Lamellibranehiata as have the power of moving from place to 

 place, by the protrusion of their distensible ' foot/ do so in the 

 direction which the words ' anterior^ and ' posterior,^ as used here, 

 imply. The shell of the Anodon is, as in the immense majority 

 of Lamellibranehiata, nearly or quite equivalve, whilst, as in all, it 

 is inequilateral. By its size, which is in an inverse ratio to the 

 extent of junction of the lobes of the mantle which secretes it, 

 it surpasses all European fluviatile bivalves, and sections, or indeed 

 fragments of it, are consequently exceedingly instructive, as to the 

 structm'e of the various layers of which the shell is composed. 

 Nearly the whole of the inner surface of the shell possesses an iri- 

 descent appearance, due, probably, mainly to the light being dif- 

 fracted by the layers of delicate lamellae with irregularly overlapping 

 edges, which make up the nacre or inner layer of the shell, and are 



