424 Notes on the Classification of the Corhiculadw, etc. 



It corresponds to the slight sinuosity seen in the shell at the 

 posterior termination of the palleal line. 



§ IY. — The syphons. The genus Corbicula is provided 

 with genuine syphons ; these organs are, however, unusually 

 short. Considering them from the branchial chamber, they 

 have the appearance of two circular apertures, of equal diame- 

 ter, placed one above the other. Examining them in the rear 

 cavity of the syphons, they resemble two vertical button-holes, 

 separated by a horizontal plait ; the inferior syphon (the bran- 

 chial) is somewhat longer than the superior (the anal) syphon. 

 In both syphons the edges are approximate, and are furnished 

 with conical tentacles. The tissue consists of longitudinal 

 and circular fibres. In all probability, when the animal is 

 alive, the syphons are capable of being projected to the extent 

 of several millimetres. The syphons are invariably separate. 



§ V. — The breathing system. The gills, or branchiae, are 

 quadrangular, and are elongated from the anterior to the poste- 

 rior side. Owing to the external gill being less high than the in- 

 ternal one, the latter projects forward and below. A narrow pro- 

 longation of the external gill is formed on the oblique dorsal 

 line by which the internal and external gills are joined. This 

 branchial appendage exists in a great number of Acephala ; we 

 have noted it in Petricola, Capsa, Galatea, etc. It is found 

 also in the Acephala, which have but one external gill. 



In Corbicula, the superior portion of the external gill is less 

 developed than in Galatea, in which genus it occupies more 

 space than a third of the inferior portion. 



The edges of the gills are free and are not soldered to those 

 of the opposite side, as is generally the case in the larger num- 

 ber of the genera of " Les Conques " (Capsa, Petricola, 

 Yenus, etc.), and as is the case also in Galatea, though in the 

 latter genus the extent of the surface soldered is very limited. 



The folds of the gills are quite broad, and recall those of 

 Petricola, Galatea, etc. 



§ YI. — The digestive system. The palpi are triangular, 



