SPIRIFERiE OF THE HAMILTON GROUP. 217 
prevailing form of the sinus is shallow and rounded in the bottom : it 
is sometimes flat and sometimes with a fold in the centre (See figure 
11), and again it is angular (as in figures 19 and 20). 
Dorsal valve moderately convex, sometimes becoming gibbous. The 
sides are gently curving, and usually flattened towards the cardinal 
margin : the mesial fold prominent and well defined, flat or rounded 
above, sometimes with a median groove and again angulated in the 
middle. The beak is incurved, and the area extremely narrow, about 
one-third as high as that of the ventral valve. 
Surface marked by from eight or ten to twenty or more subangular pli¬ 
cations on either side of the mesial fold and sinus : the plications are 
not very prominent but usually well defined, the outer half of the 
number not reaching the beak, but terminating in the callosity along 
the area-margin. The plications are crossed by numerous fine lamellose 
striae, which become'crowded together and closely imbricating towards 
the front of the shell, sometimes presenting several interrupted lines 
of growth. 
The proportions are extremely variable, the length being in some 
specimens about two-thirds af great as the width ; while in others more 
extreme, the width is three inches or more, measured on the hinge-line, 
and the length is scarcely three-fourths of an inch. In one specimen 
before me, with a width rather more than four inches on the hinge-line, 
the length- is less than three-fourths of an inch : this great lateral exten¬ 
sion is not due to age. Some of the younger shells are extremely mucro- 
nate, as shown in figure 3; while in figures 1, 8, 10 and 11, showing 
gradations in size, there is comparatively little extension on the hinge¬ 
line. 
The interior of the ventral valve shows short and rather strong teeth, 
with scarcely any extension of the dental plates, and a small striated 
muscular area, in the centre of which are the elongate occlusor muscular 
markings. 
In the dorsal valve, the cardinal process or callosity is well preserved, 
[ Paleontology IV.] 28 
