DA MAZATLAN BIVALVES 
Genus SPHENIA, Turton. 
For Monograph of this genus, with amended generic characters, 
see A. Adams in Proc. Zool. Soc. 1850, p. 86. See also Forbes 
& Hanl. Br. Mol. vol. 1. p. 189—193 :—Clark Br. Mar. Test. 
Moll. p. 150. 
30. SPHENIA FRAGILIS, 2. s, 
S. animali in eryptis latibulante, erge varie distorto ; testa 
parva, tenui, subnacred, via rugose striata ; epidermide fusco- 
virente copiose indutd, rugarum increscentium concentricarum 
plend, postice in siphone longd porrecta: parte posticd plus 
minusve subcarinatd ; valva sinistra dente ligamentum ferente. 
plus minusve seu prolongatd seu extante; dextra alveo conveniente. 
nonnunquam denticulo subextante: vinpressionibus muscularibus 
subrotundatis, sinu pallit lato, rotundato, haud alto. 
Tt is surprising how much of the very minute description of 
S. Binghami given in Forbes & Hanl. Br. Moll. i. 191-2, applies 
exactly to individuals of this species. Indeed, if young speci- 
mens of the two were mixed together, I should hardly know 
any sufficient ground of specific distinction, except in the texture 
which is more nacreous, and the pallial sinus which is broad, 
though shallow. The young shells can sometimes be told from 
those of Saxicava arctica only by the hinge, as in Binghami: 
and there is often seen the little denticle by the hgament pit 
noticed by Turton, not Hanley, and conspicuous in young 
shells of Spheenia Binghami in my possession, nestling in cre- 
vices of limestone dredged off Weymouth. Like other nestlers 
(anlike the true borers, which are moderately constant in form) 
it is extremely irregular. Many well characterized species 
might be made out of extreme forms ; but unfortunately 
for the lovers of multiplication, individuals were sufficiently 
numerous to supply many connecting links. The normal state 
appears to be not very inequilateral and tolerably well rounded : 
the shell is then shaped somewhat like Psammobia: but it is 
generally more or less produced, when the posterior portion 
becomes marked off by an angle, in very long specimens 
amounting toa keel, sometimes with a trace ofa double one. 
When it lives in dead Balani &c., 1t becomes very short, inflated 
and gibbous, resembling Corbula or sometimes Nexra. The 
ligamental plate then becomes narrow, projecting and sinuated, 
more like the tooth of Mya. These variations are seen in the 
