ON THE STRUCTURE OF ENIGMA ^NIGMATICA. 



255 



or less symmetrical; tlie umbo is close to the dorsal margin 

 aud is more or less median in position and the ligament is 

 immediately within and below the umbo. In ^Enigma, as 

 is clearly shown in the text-figure 1, the left valve is 

 asymmetrical; the umbo is prominent and obliquely curved, 

 so that it points toward the anterior aud upper margin, and 

 it is situated at a distance of some 3 — 5 millim. from that 

 margin, but is connected with it by a narrow slit or fissure. 

 Internally this fissure extends as far as the ligament, and is 

 more or less at right angles to it. The ligament, which is 

 3 — 4 millim. long in large-sized specimens, is situated obliquely 



TjiXT-riG. 1. — The left valve of iEuigma ajnigmatica ; internal 

 view (left-hand figure) and external view (light-haud figure), a, b. 

 The true dorso-vcntral axis of the shell, c, d. The apparent dorso- 

 ventral axis. ad. Impression of the adductor muscle of the valves. 

 a.r.p. Impression of tlie anterior retractor pedis muscle, d.m. Im- 

 pression of the byssus muscle, pl.m. Impression of the branchio- 

 pallial muscle, p.r.j). Impression of the posterior retractor pedis 

 muscle. 



above the umbonal fossa, and a lino, ab, drawn at right angles 

 to it indicates the original dorso-ventral axis of the shell. It is 

 obvious that an inequality of growth, clearly indicated by the 

 growth lines on the outer surface of the valve, has produced 

 a secondary symmetry, and that the apparent dorso-ventral 

 axis has been rotated through an angle of 40° in a postero- 

 anterior direction. Sassi has shown that in Auomia ephip- 

 pium the animal has undergone a similar rotation to an 

 extent of 90°, that is to say through an angle twice as great 



