44 UNIONID.^. 



Shell oval, rather compressed, not so thin as in the usual 

 or typical form of the last species, olive-green or brown, with 

 darker transverse bands denoting the lines of growth, and 

 irregularly wrinkled in the same direction : epidermis rather 

 thicker than in A. cygnea : heahs straight, placed at a distance 

 of about one-third from the anterior end: umhonal region 

 compressed, closely plaited: ligament short and prominent: 

 upper margin or hinge -line raised into a kind of crest, and 

 curved: anterior side rounded and gaping below, with an 

 oblique slope towards the lower edge : posterior side curved 

 and abruptly sloping to a wedge-like point: lower margin 

 gently cur^'cd : inside and liinge as in A. cygnea ; but the 

 lining is much thicker in the present species, and the muscular 

 impressions are consequently much more deep and distinct. 

 L.2-1. B.3-5. 



Yar. 1. radiata. Shell (or rather the epidermis) marked 

 with green and yellow rays. 



Var. 2. ventricosa. Shell larger, more solid, exceedingly 

 tumid, especially in the middle and towards the umbonal 

 region, also marked with green and yellow rays. A. ventri- 

 cosa, C. Pfeiffer, ii. p. 30, pi. iii. 



Yar. 3. complanata. Shell oval, greatly compressed, brown: 

 heahs placed close to the anterior margin : upper margin raised 

 and curved : anterior side abruptly truncate. A. complanata, 

 (Ziegler) Rossmassler, iv. p. 24, f. 283. 



Habitat : Same as that of A. cygnea ; but it ranges 

 further to the souths being a Sicilian species. It has 

 not been noticed in this country as a tertiary fossil. 

 Var. 1. The rayed markings form scarcely a varietal 

 character, being common to half-grown individuals of the 

 last, as well as of this, species. Var. 2. R. Exe (Clark). 

 This variety lias been referred by Moquiii-Tandon 

 to A. cygnea ; but it evidently belongs to the short 

 form, or what is generally called A. anatina. None of 

 my specimens (of which I possess a series) are as broad 

 as the one represented by Pfeiffer in his figure 4. This 

 variety attains a larger size than the typical form, being 

 more than 3 inches long, 5 wide, and 2 in depth. Var. 3. 



