DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



117 



Fig. 23. Ledella aberrata. Siphonal region, as seen from the inside of the 

 mantle cavity. See abbreviations to the text-figures p. 1 02. Scale = 0. 1 mm. 



This species occurs at upper to mid slope depths in the tropical 

 western Atlantic in the Brazil, Guyana, Caribbean and Gulf of 

 Mexico Basins (Dall, 1889; James 1972). Depth range; 943-2500m. 



The holotype for T. agatheda was not designated by Dall ( 1 890), 

 nevertheless he did illustrate the left valve from Albatross Sta. 2754, 

 1 1"40'N 58"33'W (USNM 95437) which we have here nominated as 



lectotype. We have redrawn the shell and added detail of the hinge 

 plate (Figs 24 and 25). In addition, James (1972) reported that two 

 valves of T. agathedawere included in USNM 63 149 from Blake sta. 

 236, 2909m off Bequia, furthermore USNM 94326 from Blake stas 

 26 and 30 between Cuba and Yucatan, identified by Dall as Leda 

 push, are examples of T. agatheda. 



Shell description (Figs 24 & 25) 



Shell small, sub-ovate, wide, sub-rostrate, ornamented with con- 

 centric ridges, postero-lateral furrow ventral to sub-rostrum, 

 ill-defined lunule outlined by obscure ridge, escutcheon similarly 

 ill-defined and bounded by faint ridge; umbo prominant, poste- 

 rior to mid-line, inwardly directed; antero-dorsal margin concave, 

 with change in slope at anterior limit of hinge plate, slightly 

 flattened anteriorly dorsal to the anterior limit of shell, antero- 

 ventral and ventral margins smoothly curved, postero-ventral 

 margin slightly sinuate, posterior margin acutely angled, postero- 

 dorsal margin slightly concave, marked angle at posterior limit of 

 hinge plate and thereafter almost straight to form sub-rostrate 

 posterior margin; anterior and posterior limits of shell are ventral 

 to horizontal mid-line; hinge plate strong, with edentulous space 

 ventral to umbo, hinge teeth stout, chevron-shaped, up to 12 in 

 each series in shell 6mm total length; internal ligament small, 

 close to shell margin, external ligament amphidetic. moderately 

 short, stout. 



Young shells are less rostrate, with the posterior and anterior 

 limits of the shell more dorsal in position. 



Although there is variation in the shape of the shell, the height/ 

 length and the post-umbonal length/total length ratios increase 

 slightly with increasing size (Table 1 ). The maximum total length of 

 the present specimens is 6.0mm. 



Fig. 24. Tmdariopsis agatheda. External lateral view of the right valve of the lectotype and an internal view of the hinge plate of the same valve, from 

 U.S. Fish Commission station 2754 East of Tobago, USNM 95437. Scales = 1mm. 



