46 
THE GEOLOGIST. 
while the variable ones, which sometimes may be as large as the 
others, and are sometimes scarcely to be found, have a line or dot 
above the numerals to indicate that they are subordinate. Teeth 
are often cleft a little, and sometimes look like two, so that it is oc- 
casionally convenient to indicate them in the formula, which is done 
by writing a small figure ^ above the numeral, as though it were 
raised to the second power. 
Earely teeth are anchylosed, but, excepting Isocardia, only under 
the umbones, and then only at their outer edges, forming an inverted 
V shape, which is indicated by a small figure ^ placed above the nu- 
meral 2. 
Typically, teeth are in three groups : those under the umbo, called 
umbonal ; those near the front adductor muscle, called anterior ; and 
those under the ligament, called posterior. These two latter groups 
are often elongated latitudinally, and when this is the case have a 
small I added to distinguish them from the ordinary conical forms. 
The spaces between the groups of teeth on each side of the umbo 
are generally equal, and such spaces are marked by a single dot 
placed midway between them. To indicate a longer distance, two 
dots may be put one after the other, and a very short one is marked 
by two dots one above another. 
Eor a cartilage-pit the letter F is used (abb^e^ iation of fossa), and 
where a tooth has the letter/" above it, the tooth supports the carti- 
lage, as in My a, etc. 
The sign + means many ; it is chiefly used to indicate the nume- 
rous teeth of the Area tribe. The straight line running between the 
numerals marks the plane dividing the valves. The letter S is an 
abbreviation for "with a pallial sinus;" L, for lunule; E, with an 
escutcheon; O, with an ossicle; M, massive; and Q, quadrate. 
The teeth are read from left to right, the posterior end being to 
the right, — so that the lower valve is not the left. 
As an illustration of the way these symbols are used, an example 
or two may be cited. 
Trigonia. — Here there are in the lower valve two leeth under the 
umbo, and no anterior teeth, so the number 2 is written under a line 
and with a dot on each side to show that the teeth are umbonal, 
thus .-g-. . Then in the upper valve there is on the left a single 
tooth, and to the right of it two anchylosed ; and as these are under 
the umbo, they, too, are written with the dot on each side, thus : 
•1:2^'. 
— '■ — . Putting these valves together, the generic characters for 
• 1 • 2^ • 
Trigonia will be ^ — ; , and as no other genus has teeth quite like 
it, obviously no other character need be mentioned. It is found con- 
venient, iu remembering these formulae, to prefix the initial letter of 
the genus. 
Ceassatella. — Here the upper valve has two teeth and a cartilage- 
pit under the umbo, and posteriorly one tooth ; so the dentition will 
