8 ACAM II"' HITES. 



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

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

 (-4./ . The pores are normally 18 in number, but rarely 



oner or two additional pores are developed behind the posterior 

 valve. 



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



iiii: >f tin* valves, ami the denticulation of the posterior valve. 



of tin-in 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 

 - 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 tegnientum. 



l>. 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. 



!>!>!>. 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, Oryptoconchus, 

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

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

 -mall pore tufts, Loboplax. 



Section Acanthochites Risso, 1826. 



Risso (as of Leach ms.), Hist. Nat. de PEur. Merid., 

 iv, p. *J68 (first species A. communi8,= t A. discrepant; second 

 >i>ttii.<=A.fascicuIari3~LS). GRAY,?. Z. S. 1847, pp. n'i;. 

 69. AcatitlK.>- : LBAOH (MS. 1819) in GRAY, P. Z. S. 1847, p. 

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

 'fi'.rlufnn II I:I:I:M \\N-I.N. Indicia Generum Malarozoorum 

 IVim<.rdia. i. p. L> .'vid. ioid.. " Ai-mit/tochitns "). Ac<inth(n-h>f<iii of 

 CARTI N - i i: and many modern authors. AcanthochitUs CoSTA, 

 Faun. II'i_ r . Nap., p. -. /' ffura GuiLDING, Zool. Journ. v. 



