8 ACANTHOCHITES. 



the girdle-pores seem to be a constant generic character ; and even 

 in this genus some species have them very small, or even subobsolete 

 (A. floridanus). The pores are normally 18 in number, but rarely 

 one or two additional pores are developed behind the posterior 

 valve. 



A number of sections have been proposed, based on the degree of 

 covering of the valves, and the denticulation of the posterior valve. 

 Part of them have been ranked as genera by authors, but the more 

 I see of the species, the less rank I am disposed to accord these minor 

 groups. Their differential characters are trivial, and intermediate 

 forms may be expected to occur between any of them. 



Synopsis of sections, 



A. Anterior valve without radiating ribs; not obviously lobed 

 around the lower edge of tegmentum. 



b. Tail valve with one slit on each side, and a wide, shallow 

 sinus between ; girdle covered with spicules and having well 

 developed tufts, Acanthochites. 



bb. Tail valve with several slits behind ; girdle encroaching at 

 sutures on the valves. Notoplax. 



bbb. Tail valve with several slits ; girdle naked, leathery, covering 

 the valves except for a linear band at the ridge ; tufts small, 

 the pores sometimes raised on tubercles, sometimes sub- 

 obsolete, hardly visible, Cryptoconchus. 

 AA. Anterior valve having 5 radiating ribs, its lower margin 

 5-lobed ; tail valve multifissate ; girdle nearly naked, having 

 small pore tufts, Loboplax. 



Section Acanthochites Risso, 1826. 



Acanthochites BJSSO (as of Leach ms.), Hist. Nat. de 1'Eur. Merid., 

 iv, p. 268 (first species A. communis,==^ A. discrepans ; second 

 species A. carinatus=A.fascicularis L.). GRAY, P. Z. S. 1847, pp. 66, 

 69.Acanthochetes LEACH (MS. 1819) in GRAY, P. Z. S. 1847, p. 

 169 ; Guide, p. 186. SOWB., Conch. Man., edit. 2, p. 57 (1842). 

 Acanthochiton HERRMANNSEN, Indicis Generum Malacozoorum 

 Primordia, i, p. 2 (vid. ibid., " Acanthochitus"). Acanthochiton of 

 CARPENTER and many modern authors. Acanthochistes COSTA, 

 Faun. Reg. Nap., p. 2. Phakellopleura GUILDING, Zool. Journ. v, 



