178 Couthouy’s Description of 
NucvLA NAVICULARIS. 
Plate IV. fig. 4. 
A. testa test parvá, levi, ‘fragili, ovali, subequilaterali, luteo-virescenté,. 
anticé rotundatá, posticé truncatulà, cardine dentibus octodecim, intis 
albo-nitescenté.. , 
DESCRIPTION. Shell small, oval, thin and. brittle, 
rather compressed, sub-equilateral, slightly gaping at both 
ends, smooth and polished,, with occasional eroded 
patches toward the summits ; color a pale greenish yel- 
low; strie of growth concentric, regular and very in- 
distinct ; the anterior extremity is elliptically rounded, 
the basal margin curved strongly outwards, the posterior 
extremity is somewhat narrowed and compressed, feebly 
rostrated and very slightly truncated ; the beaks are 
quite prominent, inclined a little backwards, and con- 
tiguous at their summits ; hinge furnished with sixteen 
or eighteen aculeated teeth, divided in the middle by a 
spoon-shaped fossa projecting obliquely forward, the an- 
terior side having usually two or three more teeth than. 
the other. "The interior is white and polished ; muscu-. 
lari impressions remote; nearly imperceptible, ihe ante- 
rior oblong, narrow, the posterior round and very small. 
Length eleven-twentieths, height five-twentieths, and 
diameter three-twentieths of an inch. — . 
Inhabits Massachusetts Bay, vicinity of Plymouth. 
OssrRvATrIONs. Somewhat resembles N. rostrata, 
Mont., but is destitute of the sulciform stris so remark- 
able in that shell, and is less prominently rostrated. It 
is distinct from N. myalis, Nobis, by the different po- 
em of the — and the more strongly curved outline 
