OOLITIC SYSTEM OF SCOTLAND. 369 
Isastrea of Helmsdale, whose resemblance to Jsastrea oblonga 
I have pointed out, I remarked that not a few of the larger 
specimens had been perforated, apparently in the recent state, 
by circular openings, resembling those made by our recent pho- 
lade, and which were usually filled up by a grayish-colored 
grit, identical with that which formed the uniting cement of the 
conglomerate in which the corals occurred. I at once attrib- 
uted their formation to lithodomous shells of the QOolite, and 
ventured to describe, in the Witness for 1843, one of my first- 
found specimens as “a curious fragment of coral perforated by an 
ancient pholas.” “The cavtiy,” I continued in my description, 
“exactly resembles those cavities of the existing lithodomous 
shells which fretted so many of the calcareous masses that lay 
scattered on the beach on every side of the specimen; but it is 
shut firmly up by the coarse gritty sandstone in which the coral 
itself had lain buried; and a fragment of carbonized wood lies 
embedded in the entrance. The cave is curtained across by a 
wall of masonry immensely more ancient than that which con- 
verted into a prison the cave of the Seven Sleepers.” Several 
years, however, elapsed from this time ere I succeeded in de- 
tecting the shells by which the cavities had been formed; and 
not until two years ago did I find specimens sufficiently entire 
to admit, and that still but imperfectly, of description. They 
seem to have been slim wedge-shaped bivalves, greatly resem- 
bling modiola, but belonging evidently to the genus Lithodomus. 
Sir Roderick Murchison, in his great work on the Geology of 
Russia, figures a Lithodomus of the Oolite of that country un- 
der the specific name Lithodomus Hramanus ; but it is a greatly 
smaller shell than the Scotch one, measuring little more than 
a quarter of an inch in length; whereas the Helmsdale spe- 
cies measures, in my larger specimens, two inches and a line in 
length. The Russian species, however, in proportion to its 
general size, seems to have been a massier and broader shell. 
