38 Guide to the Mollusca. 



Order 1. PROTOBRANCHIA. 



{Case 137.) 



In the Protobranchia the individual filaments of the gills are 

 not doubled, and are arranged on their axis in two divergent 

 rows. The gills thus have essentially the same structure as those 

 of the Polyplacophora and the Rhipidoglossa. The foot is 

 flattened underneath (another primitive character), and the 

 byssus is but poorly developed. There are only a few families 

 of living forms, of which the Nucvlidae (Case 137) are the best 

 known. A number of fossils, chiefly from the Silurian, have been 

 referred to this group on the evidence of the shell. 



♦ 



Order 2. FILIBRANCHIA. 



(Cases 137 to 153.) 



To the Filibranchia belong some familiar molluscs, such as 

 the Scallop and Mussel. In these Lamellibranchs the two rows of 

 filaments in each gill are parallel to each other and their ends 

 are 'reflected', i.e. the filaments are bent on themselves so as 

 to make a narrow V. Each series of filaments, from the gill-axis 

 to the bend, is named the descending lamella, and from the bend 

 to the tip the ascending lamella. The adjacent filaments are 

 locked together by tufts of cilia which fit into each other like 

 two hair-brushes pressed together by their bristles. The foot 

 usually exhibits a well-developed byssus. 



The Anomiidae are remarkable for a deep notch in the right 

 valve, through which the byssus passes when the animal is fixed 

 down. The Placunidae (Case 138) have a very flat pearly shell 

 and a unique hinge consisting of two long divergent teeth. The 

 young shells are so transparent that they have been largely used 

 in the East for glazing windows. 



The Arcidae have strong ponderous shells with numerous small 

 teeth in the hinge-line. A few of the smaller species live on corals. 



The Trigoniidae (Case 141) include a few living species from 

 Australia and more than a hundred forms from Jurassic and 

 Cretaceous strata. 



The Mussels (Mytilidae, Cases 142-145) are well known. The 

 byssus is usually well developed ; with it the animals attach 

 themselves to rocks or to one another, often forming large colonies. 

 The coloration of certain Mytilidae is very attractive, rich brown 

 and green being the prevailing hues (cf . Case 143). The species of 

 Lithodomus are not fixed down by a byssus, but burrow into coral 

 and limestone. 



Inoceramus, found fossil in the Jurassic and Cretaceous, is 

 remarkable for its gigantic size. A species obtained in Nebraska 



