THE HAMILTON ASSOCIATION. - 85 
sandstone—that portion of the Devonian formation supposed to be 
chiefly of laenstrine origin. Externally it bears a remarkable resem- 
blance to to the Unionide of our lakes and rivers, known better here 
as Clams. I do not know whether it presented the usual nacreous 
lustre of the family or whether it had been submitted to microscopi- 
cal examination by the late Dr. Carpenter. Perhaps not; for 
Woodward in a note, ‘f Manual of the Mollusca,” page 213, remarks: 
“‘ The fossil shells of the older rocks are so generally pseudomorphous 
or partake of the metamorphic character of the rock itself, that it is 
difficult to obtain specimens in a state for microscopic examination.” 
Here a strange coincidence occurred. On putting aside my 
notes, I received a package directed from Ottawa. On removing 
the outer cover, I found ‘‘ Notes on the discovery of large Unio-like 
shells in the Coal Measures, South Joggins, N.S ,” by J. F. Whiteaves. 
Doubts may be entertained regarding the true affinity of the Irish 
specimens, but taking into account that the Nova Scotia shell was 
associated with true land plants, (fragments of Sigillaria, Cordaites) 
and that a portion of the test of a bivalve (Nacreous throughout) 
was found in the same bed, few can doubt but Professor Whiteaves’ 
conclusions are pretty accurate (despite absence of the hinge 
dentition) viz., that it represents an aberrant and extinct type of the 
Unienide. 
THE ARK SHELL. 
Another modern Lamellibranch, named above, had a repre- 
sentative in the Paleozcic age, the Crenodonta, of Salter. The hinge 
line displayed a double row of teeth, bent, connected by smaller 
ones below the beak. This peculiarly was unknown to Dr. Jas. 
Hall, and, from its external appearance solely, he had previously 
named the fossil Ze//nomya, as its shape suggested a certain rela- 
tionship to the Zeé/zns and Myas. Hall’s name apparently holds its 
original place in the States. Dana figures Z. WVasu¢a H., Trenton, 
at page 200 in his manual (revised edition), and also gives the teeth, 
which clearly shows that it belongs to the family Avcactde. The 
Isoarca, of Count Munstus, 1842, antedates both names, and Wood- 
ward, in the manual of the Mallusca, includes Salter’s Ctenodonta 
. under the head. 
‘“The ark shells of the Paleozoic strata,’ he remarks, “ have 
their anterior teeth more or less oblique like Arca, their posterior 
