DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



15 



Fig. 4 Yoldiella lucida. Hinge plates of a, right valve of a specimen from Sta. 105 (North America Basin); b & c, right and left valve of 

 specimens from Thor' Sta. 28 to show variation in the shape of the ligament. (Scales = 0.5 mm). 



specimens the antero-dorsal margin is more convex, the 

 postero-dorsal margin is normally angulate and the posterior 

 margin more rounded (Fig. 4). As in many species of 

 Yoldiella older specimens tend to become subrostrate. 



Prodissoconch length: 198 p.m. Maximum recorded shell 

 length: 6.9 mm. 



Internal morphology. (Figs. 5 & 6) The mantle margin 

 has a well-developed anterior sense organ. Posteriorly the 

 siphons are combined, the inhalent siphon being open ven- 

 trally. A well-developed feeding aperture lies immediately 

 ventral to the siphons with numerous gland cells present in 

 what is a broadened region of the inner muscular lobe. A long 

 siphonal tentacle originates usually on the left side of the 

 siphonal embayment near the base of the siphon. The adduc- 

 tor muscles are large and unequal in size. The posterior 

 muscle is oval in cross-section and between 1/2 and 1/3 the 

 size of the anterior which is more circular in section. 



The visceral and cerebral ganglia are relatively large, 

 club-shaped, with an exceptionally thick connecting commis- 

 sure. The pedal ganglia are also large but more round in 

 outline, and each with a large statocyst dorsal to it. The foot 

 is anteriorly directed with well-developed pedal retractor 

 muscles. The byssal gland is moderate in size. The gills are 

 well-developed with between 16-23 gill filaments, the num- 

 ber depending on the size of the individual. Posteriorly the 

 gill axes attach to the junction between the two siphons. The 



labial palps are moderately large extending approximately a 

 third across the body with between 13-23 palp ridges on the 

 inner face. The palp proboscides are well-developed and are 

 long and muscular. A wide ciliated oesophagus opens into a 

 large stomach, the dorsal hood of which lies close to the 

 dorsal margin of the body. There is a large style sac which 

 penetrates the lower posterior half of the foot. The hind gut 

 forms a single loop on the right side of the body. It has a 

 typhlosole along its entire length. There are two digestive 

 diverticula to the left and one on the right of the stomach. 

 Material similar to that present in the stomach was observed 

 in the left hand digestive diverticulum. The kidney is rela- 

 tively small in comparison with Yoldiella species from deeper 

 waters. The sexes are separate, and the gonads overlie the 

 viscera dorsally and laterally. 203 ova were present in a 

 specimen of 3.4 mm total length. 



Yoldiella obesa obesa (Stimpson 1851) 



Type locality. Original not known; Type locality here 

 designated, St. Georges Bank, U.S. Fish Comm. Sta. 2072, 

 2.10.1883, 43°53'N, 65°35'W, Beam Trawl, 858 fms. 



Type specimen. Holotype believed lost in Chicago fire. 

 Neotype here selected, USNM No. 38419. 



Leda obesa Stimpson 1851, p. 113; Stimpson 1851, p. 10, pt. 

 II, Fig. 1; Tryon 1873, p. 184, pi. 38, Figs. 500, 501. 



