OOLITIC SYSTEM OP SCOTLAND.

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Isastrea of Helmsdale, whose resemblance to Isastrea oblonga Ihave pointed out, I remarked that not a few of the larger spe-cimens had been perforated, apparently in the recent state, bycircular openings, resembling those made by our recent pho-ladse, and which were usually filled up by a grayish-colouredgrit, identical with that which formed the uniting cement ofthe conglomerate in which the corals occurred. I at onceattributed their formation to lithodomous shells of the Oolite,and ventured to describe; in the Witness for 1843, one of myfirst-found specimens as “ a curious fragment of coral perfo-rated by an ancient pholas.” “ The cavity,” I continued inmy description, “ exactly resembles those cavities of the ex-isting lithodomous shells which fretted so many of the cal-careous masses that lay scattered on the beach on every sideof the specimen ; but it is shut firmly up by the coarse grittysandstone in which the coral itself had lain buried; and afragment of carbonized wood lies embedded in the entrance.The cave is curtained across by a wall of masonry immenselymore ancient than that which converted into a prison thecave of the Seven Sleepers.” Several years, however, elapsedfrom this time ere I succeeded in detecting the shells bywhich the cavities had been formed; and not until two yearsago did I find specimens sufficiently entire to admit, andthat still but imperfectly, of description. They seem to havebeen slim wedge-shaped bivalves, greatly resembling modiola,but belonging evidently to the genus Lithodomus. Sir Ro-derick Murchison , in his great work on the Geology of Rus-sia, figures a Lithodomus of the Oolite of that country un-der the specific name Lithodomus Eramanus; but it is agreatly smaller shell than the Scotch one, measuring littlemore than a quarter of an inch in length; whereas the Helms-dale species measures, in my larger specimens, two inches anda line in length. The Russian species, however, in propor-tion to its general size, seems to have been a massier and