Notice of the Nucula decussafca found at Leith. 107 
of artificial soil, the continuation of the sand and gravel bed 
was observed to cover the boulder clay at six or eight feet 
above high-water level. In a section made for the foundation 
of the east wall of the building, the boulder clay was cut down 
to the depth of 12 or 14 feet ; and in the sand and gravel-bed 
overlying the clay at this part, amongst other shells usually 
found in this bed, a detached right valve of a Nucula was 
obtained, having the sculpture and characters of Nucula de- 
cussata. The epidermis was well preserved, and of an olive- 
green colour. It was marked with concentric wrinkles, and 
strongly raised radiating striae, with short but distinct trans- 
verse plicae on the dorsal area. The inner surface was pearly 
white, and had a crenulated margin, with twelve pectinated 
teeth in front of the cartilage-depression, and about double 
that number behind. The valve was rather more elongated, 
and larger than the ordinary examples of the common Nucula 
nucleus. 
The N. decussata is not recorded as having been found in 
the Firth of Forth, but it lives on the west coast of Scotland, 
and in the Hebrides, and is said by Loven to be a Swedish 
shell. The specimen was accidentally broken, and the fact of 
the valve having been found is only recorded to direct atten- 
tion to a closer scrutiny of the marine contents of this depo- 
sit, especially in sections inland from the present high-water 
level ; as the lateral extent of this so-called raised sea-beach 
bed, and its relation to other accumulations of a similar litho- 
logical structure, but destitute of any trace of marine remaiuvS, 
had not been satisfactorily determined. 
IV. Contribution to a Monograph of Iceland Spar. By Alexander 
BiiYSON, Esq. Part I. 
This communication will be given complete (along with 
Part 11. ), in a future fasciculus of the Proceedings. 
V. On a Method of constructing Polarizing Prisms of Nitrate of 
Potash. By T. Strethill Wright, M.D. 
Dr Wright stated that many doubly-refracting substances, 
when immersed in dense and transparent fluids, had, as was 
well known, the property of polarizing light. This polariza- 
