ON THE STRUCTURE OF ENIGMA ^NIGMATIOA. 261 



ciliated but not glandular. The outer right labial fold, con- 

 tinuing to increase in vertical depth, eventually passes into 

 the upturned reflected lamella of the right external demi- 

 branch, and the internal right labial fold becomes continuous 

 with the thin membrane by which the direct lamella of the 

 right internal demibranch is attached to the body-wall below 

 the byssus muscle. The right labial groove thus becomes 

 continuous with the inter-branchial chamber of the right side, 

 but it is continued backwards as a cul-de-sac for some dis- 

 tance beyond the point of union with the gill, and in sections 

 the right branchia appears to be suspended from the lower 

 wall of this cul-de-sac, as is shown in text-figure 2, e. 



The foot (fig. 1,/.) is reduced to a flat muscular projection 

 at the anterior angle of the byssus cavity. In most of the 

 spirit-preserved specimens it is contracted to a small, lanceo- 

 late, muscular mass, but in the individual figured it is unusu- 

 ally long and ribbon-shaped. The extremity of the foot is 

 always pointed, and bears a single small tentacle, similar in 

 all respects to the marginal tentacles of the mantle. There 

 is no infundibuliform cavity at the end of the foot as in 

 Anomia ephippium, but the right (morphologically the 

 ventral) surface is grooved and covered with numerous trans- 

 verse, ciliated ridges. The mass of the foot is highly muscular 

 and contains numerous mucous glands, and it is evident tliat 

 this organ is very extensile. It seems probable that it can 

 be protruded some distance beyond the shell, and that it is 

 auxiliary to nutrition, minute particles being swept by ciliary 

 action along the groove on its right surface, and thence to 

 the right labial groove. 



The principal muscles connected with the foot have not 

 undergone as much modification in .Enigma as in Anomia 

 owing to the lesser degree of rotation in the former genus. 

 The two tapering muscular bands running respectively for- 

 wards and backwards from the great retractor muscle of the 

 byssus to their surfaces of attachment on the left valve, are 

 clearly the homologues of the anterior and posterior retractors 

 of the foot of other Lamellibranchia (tig. 2, a.r.p. siud p.r.p.). 



