264 GII,BERT C. BOUKNE. 



can find no ti'ace of them, and am certain that they do not 

 exist. It should be noted that the velar flap on each side 

 abuts on the thickened and corrugated lower moiety of the 

 mantle lobe, so that ciliated discs on that side are un- 

 necessary. 



As in An o mi a ephippium, there are no ciliated discs on 

 either the direct (descending) or reflected (ascending) limbs 

 of the filaments, but there is a well-developed single row of 

 discs at the angle of reflection, the details of which are 

 shown in fig. 5. There is, as in A. ephippium, a very low 

 inter-filamenter septum close to the angle, but beyond this 

 no other iuterlamellar junctions whatever. A transverse 

 section of two filaments, showing the frontal and lateral cilia 

 and other details, is given in fig. 7. The attachments of the 

 gills to the body-wall and mantle are affected to a considerable 

 extent by the asymmetry of the anterior half of the body 

 and by the fact, mentioned above, that the visceral mass is 

 adherent to the left mantle lobe in the anterior part of the 

 body but to the right mantle lobe in the posterior part. As 

 these relations have not been sufficiently fully described in 

 Anomia, I will enter into them somewhat closely in 

 j^nigma. The posterior recurved ends of the gills lie free 

 in the mantle cavity and are not attached to the mantle. In 

 the more anterior part of their courses the right and left 

 branchiae are differently affected by the asymmetry due to 

 the twisting of the byssus and foot over to the right. The 

 relations of the left branchia are on the whole those of a 

 normal symmetrical Lamellibranch. At a point nearly ver- 

 tically below the anus its axial fold becomes attached by a 

 deep suspensory fold to the left mantle lobe. In this fold 

 run the muscular fibres which have already been described 

 as a specialization of the pallial musculature forming a 

 retractor muscle of the branchia (figs. 2 and 13). Passing 

 forward the axial attachment of the left branchia becomes 

 shifted more and more towards the middle line, largely in 

 consequence of the interposition of the fibres of the posterior 

 retractor pedis muscle between it and the mantle (fig. 12). 



