BIVALVE SHELLS. 



467 



left, as the ligament may casually loosen, exhibiting the form 

 of a somewhat elevated transverse tooth. It is of an oblong 

 heart-shaped figure, appearing like a piece of enamel fixed to 

 the point of junction between the valves, with the pointed 

 end directly under the beaks."* — The internal hinge carti- 

 lage of the genus Cleidotha^rus has an elongated testaceous 

 appendage resembling the human clavicle in its form. It is 

 called the clavicle by Mr. S. Stutchbury, but it may be re- 

 duced to the ossicle. 



An edentulous hinge is sometimes a merely thickened cal- 

 lus for the insertion of the ligament ; at other times there is 

 a fossa, or p'/t, excavated under the beaks. This is named 

 the scrobi cuius, by Linnaeus. It is often marked across with 

 lines or striae, as in the Oyster. 



In the genus Pholas there arises, from the inner part of 

 each valve just under the umbo, a long sickle-shaped process, 

 which maybe called the falciform process. It has been often 

 called a tooth, and is considered to be a modification of the 

 tooth by Deshayes, but it is not placed similarly to the teeth 

 of other bivalves. Their teeth is represented in Pholas by 

 one or more ribs on the hinge margin, which answer the 

 same purpose as teeth in other shells, keeping the valves in 

 their relative position one with another. 



Fiff. 88. 



If you examine the internal surface of a bivalve you may 

 notice that it is impressed more or less distinctly, with a 

 roundish spot on each side (Fig. 88, a, a), with a narrow 

 border parallel to the lower margin (Fig. 88, b, h), and with 



* Conchyli.a Insul. Brit. 34. See, also, Encyclop. Meth. Vers. ii. 37. 

 Deshayes, Trait. Elein. de Conchyologie, i. 205. 



II n 2 



