SPIRIFER^ OF THE HAMILTON GROUP. 317 



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. 

 SuEFACE 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 hy numerous fine lamellose 

 stria), 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 as 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, 



[ PiLiBONTOLOGV IV.] 28 



