189 
T eii b rr ATiJLyE of the plaited sort seem to form a division, 
but in the great variety of species the passage from the 
smooth to the plaited ones will be so gradual, that it will 
be hardly possible to draw a line D I have put a few plaited 
ones together here, and once thought they might be named 
numerically from the ridges, but I now expect that will 
be equally difficult. I have taken such means as were 
most convenient for the present, hereafter perhaps we may 
improve. The species here represented and those figured 
in tab. 15, agree in many particulars: the perforation in 
both is at the end of the beak of the shallow valve, and 
occasionally they have both two oblique sulci or narrow 
furrows, that would if imperfect imply a triangular foramen, 
see fig. 8 ; the sides of the hinge are not straight in either. 
Fig. 2* shows the cast of the triangular organization; 
I shall explain this further when I figure species in which 
1 have seen the internal structure. 
T EREBR ATU LA lateralis. 
TAB. LXXXIII. — Fig. 1. 
Spec. Chau. Oval,, broader than long,, gibbous; 
middle of the front much elevated with three 
deep but short plaits; sides with two plaits 
eachj much below the middle. 
The three plaits in the middle of the imperforated valve, 
though not continued far into the shell, produce three very 
