96 SPECIES OF THE SYSTEMA. 
‘Museum Ulrice,’ the “altera testa minor” of that work leads 
us to infer that some inequivalve species must have been the 
original of that description. 
Arca Decussata. 
The example (Reeve, Conch. Icon. vol. i. Pect. f. 24) marked 
for this species in the Linnean cabinet is the Pectunculus pen- 
naceus of Lamarck, who had queried the identity of his shell 
with the Arca decussata. ‘The more ample description in the 
‘Museum Ulrice,’ where the somewhat puzzling “ rima clausa” 
is explained by “nates parum retrorsum flexe. Area inter- 
jecta clausa,” indicative probably of the peculiar absence of a 
ligamental excavation (area) at the end of the beaks, agrees 
very fairly with the features of the shell. Our author acted 
wisely in refraining from citing an illustrative figure, for at 
that period no recognisable delineation was published, and a 
reference to any approximating species would only have misled 
his readers. 
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Area pallens. 
No additional particulars respecting this ambiguous species 
are recorded in the Linnean copy of the ‘Systema.’ Our 
author, moreover, did not himself possess the shell, but from 
the first referred (ed. 10) to the then unpublished ‘Museum 
Ulrice’ for a detailed account of it. It is somewhat curious, 
then, to find a discrepancy between these two descriptions. 
The beaks, which in the ‘Systema’ are stated to be recurved, 
are declared in the ‘Museum’ “ad nullum latus obliquate.” 
Assuredly, however, the latter statement must be preferred, 
since the pallens of the ‘Systema’ is far too briefly charac- 
terised to be recognisable. All conchological writers agree in 
regarding it as a member of the modern Pectunculus, but the 
exact species seems still problematical. The shell suggested 
and delineated for it by Schréter (Einleit. Conch. vol. 11. pl. 9, 
f. 1), and usually accepted with that ready credence common to 
