1)4 The Ai)ieri.c(in Geologist. February, 1897 
{interior to which are divaricators, and to either side the ped- 
icle adjustors (see fig. 18, pi. IV). 
The muscle scars vary in size and position in individual 
specimens, and the trailing of one over the other at an une- 
qual or changing rate and in slightly different directions pro- 
duce individual differences that are often far more conspicu- 
ous than the real varietal or specific ones. The depth of the 
scars is greater in periods of retarded growth of the animal 
and is variable. Vascular sinuses are rarely seen and ovarian 
spaces are rare on the ventral valve. The foramen is a trian- 
gular delthyrium. The corresponding notch in the dorsal 
valve is nearlj^ blocked by the cardinal process. 
The exterior surface of the valves bears plications and they 
are characters, as said, of the epidermal shell layer. The}' re- 
main nearly uniformly coarse after having attained a certain 
size, and increase in numbers, their growth being thus the re- 
verse in manner from those of Orthi.s, in which the number of 
plications remains constant while the size increases. The few 
primary simple plications of small valves increase, by branch- 
ing, i. e. along the side of a large plication begins a mere 
raised line, which enlarges gradually to full size. The one 
valve is the reverse of the other, inasmuch as the plications 
of the one correspond to interplications on the other. 
As to the distinction of varieties and species the internal 
characters of this group of shells are not more constant than 
their mere outward form and plication, and there is no reason 
why the latter conveniently accessible characters could not be 
used alone for identification, and especially so since the exter- 
nal and internal markings will be found to vary dependently. 
The external characters are even the more reliable, because 
more often accessible for study than are the interiors, and be- 
cause the latter are confused from the crossing and trailing 
of one scar over the other and by differences due to age of the 
individual and to interrupted growth periods, while the exter- 
nal characters suffer only from injury to cardinal angles and 
from absorption of the outer shell layer from the older sur- 
face of the shell, and in such manners as are easily accounted 
for. The muscle scars, too, on separated valves are not rare- 
ly changed to an unnatural outline by maceration and again 
by weathering, and such changes are not so easily detected as 
obliteration of the plicated outer surface. 
