l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



Genus XENAMBONITES Cooper, 1956 



Xenambonites cf . X. revelatus Williams 

 Plate 2, figures 21-26 



Xenambonites revelatus Williams. Williams, 1962, Geol. Soc. London Mem. 

 No. 3, p. 191, pi. 18, figs. 21-23, 25. 



Description. — Shell small, concavo-convex, wider than long, wide- 

 hinged with acute cardinal extremities; anterior margin apparently 

 intraplicate in young stages; valves finely costellate with concentric 

 wrinkles; sharp geniculation near anterior. Pedical valve with fold 

 that has median sulcus in early growth stages; pedicle interior not 

 observed. Brachial valve with shallow sulcus, divided by narrow 

 median fold in posterior part, poorly defined anteriorly; pair of 

 elongate pits occupies sulcus near hinge line ; interior with tentlike 

 structure described by Cooper (1956, p. 814), apparently formed by 

 fusing of brachiophores and cardinal process ; pallial trunks originate 

 beneath notothyrial structure and run nearly straight forward be- 

 neath the margins of sulcus. Largest shell about 9 mm wide (esti- 

 mated) and 3.7 mm long; geniculation crosses midline at about 

 3.2 mm. 



Discussion. — This species is tentatively assigned because only three 

 specimens are available. The complete young shell is supplemented 

 by two broken brachial valves, no separate pedicle valve having been 

 recovered. There is little question of the generic assignment, however. 



The Alaskan shells lack the relatively coarse radial costellation of 

 X. undosus Cooper, but they possess a much finer radial ornamenta- 

 tion interrupted by irregularly spaced concentric wrinkles. Such 

 wrinkles are not present in X. revelatus Williams which has a 

 brachial sulcus divided similarly by a narrow median fold. Two 

 elongate pits are located close to the hinge line in the sulcus in both 

 the Scottish and Alaskan specimens. The manner of anterolateral 

 geniculation is also similar. Where geniculation crosses the midline 

 in X. revelatus it produces a narrowly angular pinching of the shell 

 (Williams, 1962, pi. 18, figs. 21, 22). In the largest Alaskan speci- 

 men, the geniculation crosses the middle of the shell in a wider curve, 

 but apparently the individual is immature ; a more angular form and 

 greater sinuosity probably would be produced by further growth. 



Figured specimens. — USNM 145344, 145345, 145346. 



