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 Nticula 

 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 remains, 

 had not been satisfactorily determined. 



IV. Contribution to a Monograph of Iceland Spar. By Alexander 

 Bryson, Esq. Part I. 



This communication will be given complete (along with 

 Part II.), 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- 



