ON THE STRUCTURE OP iENIGMA ^NIGMATICA. 257 



smaller than in the latter genus, and the dorsal sutural union 

 of the mantle edges is considerably longer, extending from 

 the posterior edge of the ligament to the ventricle between 

 the points marked x, x in fig. 1. 



Before proceeding to a description of the different organs 

 it is necessary to point out that, in addition to the postero- 

 anterior rotation described above, the anterior half of the 

 body of yEnigma (and of all other Anomiacea) has been 

 twisted round from left to right in connection with the 

 peculiar development of the byssus and its retractor muscles. 

 The nature and effect of this torsion has already been 

 described by de Lacaze Dathiers (8) and Sassi (16) ; but a 

 repetition is not out of place, if only to save the reader the 

 trouble of reference to their papers. In a normal Lamelli- 

 branch the byssus cavity and groove are situated on the 

 posterior margin of the foot, and the retractor muscles of the 

 byssus are paired and symmetrical, passing dorsally to their 

 insertions on the right and left valves on either side of the 

 hinge. In the Anomiacea the byssus, instead of passing out 

 between the valves, passes through the well-known hole or 

 sinus in the right valve, and drags the posterior margin of 

 the foot over to the right. The retractor muscle of the 

 byssus, instead of being paired and attached near the hinge 

 line of both valves, is single and is attached near the centre 

 of the left valve. As a consequence of these displacements, 

 the whole of the anterior half of the body is twisted over to 

 the right in such a way that the paired organs of the right 

 side come to lie above the byssus muscle, and those of the 

 lef b side below it ; the visceral nerve commissures and the 

 kidneys being specially affected. When this torsion of the 

 foot and the lower part of the visceral mass is kept in mind, 

 much that is puzzling in the anatomy of the adult is made 

 clear. 



The Mantle. — The edges of the mantle are free for nearly 

 the whole of their extent, and are only united dorsad of the 

 visceral mass between the points marked x, x in fig. 1. The 

 left mantle lobe is entire, and covers in the whole lef t side of 



