22 2 AN INTRODUCTION TO THE STUDY OF FOSSILS 



left respectively, as the shell is held with beaks uppermost and 

 with the anterior extremity directed outward. 



The following are the criteria for distinguishing the anterior 

 and posterior ends of a valve. 



1. The umbos are nearly always directed forwards. 



2. The paUial sinus is posterior. 



3. The external ligament is never entirely anterior, and is 

 usually mostly posterior to the beaks. 



4. When there is but one adductor impression it is the pos- 

 terior. 



Both pelecypods and brachiopods have an external, bivalved, 

 calcareous shell. That of the former differs from that of the 

 latter as follows : — 



1. Valves are right and left, not dorsal and ventral. 



2. Valves are usually inequilateral and equivalved. 



3. Umbos are never perforated by a pedicle opening. 



4. Teeth when evident are present in both valves. 



5. Ligament is present. 



Shell accessories are present in some genera, such are de- 

 scribed under Teredo. 



The anterior adductor muscle may become diminished in 

 importance or entirely absent. In such a case the posterior 

 adductor necessarily assumes a more central position as a result 

 of the greater demand placed on it for mechanical efficiency. 

 The disappearance of the anterior adductor and the assumption 

 by the posterior of a more central position are accompanied 

 by a shortening of the antero-posterior axis of the shell and a 

 proportional lengthening of the dorso-ventral axis. Such a 

 transformation has been effected in the oyster (Fig. 95). 



Pelecypods range from the lower Paleozoic to the present 

 time, being, however, extremely rare in the Cambrian, where 

 they are represented by a very few doubtful genera, such as 

 Fordilla. Fordilla is oval, concentrically striated, and some- 

 what similar in appearance to Estheria of the Crustacea, with 

 which it is at times classed. 



