694 



CHAPTER XXXVI. 



LAMELLIBRA NCHIA T A— continued. 



DIVISIONS OF LAMELLIBRANCHIATA. 



Various attempts have been made to divide the Lamellibranchiata 

 into sections equivalent to the " orders " of the higher animals, but, 

 so far, little success has attended these efforts, and no arrangement 

 which has hitherto been proposed can be regarded as thoroughly 

 satisfactory, more especially from a palseontological point of view. 

 In what follows, the Lamellibranchiata are divided into a number of 

 sections, which it is convenient to speak of as so many "orders," 

 though it cannot be asserted that they have a value equivalent to 

 the divisions known by this title among Vertebrate animals. As 

 regards these primary sections, Dr Paul Fischer has been followed, 

 and under each section will be given the characters and geological 

 range of the more important families included in it. 



Order I. Ostreacea. 



In this order the mantle-lobes are completely free ; a single ad- 

 ductor muscle (the posterior) is alone present, the animal being thus 

 " monomyary " ; there are two branchiae on each side of the body ; 

 and the foot is wanting, or is present in a rudimentary form and 

 secretes a " byssus." The shell is inequivalve ; the hinge is eden- 

 tulous • the ligament is internal ; and the pallial line .is " entire," 

 and sometimes not distinct. This order includes the two families of 

 the Ostreidce and Anomiidce. 



Family i. Ostreid^e. — In this family the shell is generally in- 

 equivalve and fixed to foreign bodies by the substance of the left 

 valve, but is in other cases free ; the beaks are subcentral, or, in 

 many cases, twisted ; the ligament is internal, and is lodged in a 

 triangular cartilage-pit ; the single muscular impression is subcentral, 

 or excentric and posterior ; the pallial line is not distinct ; and the 



