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 2 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 1 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. 



For a cartilage-pit the letter E is used (abbreviation of fossa), and 

 where a tooth has the letter f above it, the tooth supports the carti- 

 lage, as in Mya, 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 luuule; E, with an 

 escutcheon ; 0, 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. 



Tuigonia. — Here there are in the lower valve two teeth 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 — . 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 1 * 



— '- — . Putting these valves together, the generic characters for 



• ] • 2 1 • . . 



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, in remembering these formulae, to prefix the initial letter of 

 the genus. 



Chassatella.-— Here the upper valve has two teeth and a cartilage- 

 pit under the umbo, and posteriorly one tooth ; so the dentition will 



