60 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



ALUTA ROTUNDATA (Walcott) 



Plate 8, Figures 18, 19 



Aristozoe rotundata Walcott, Amer. Journ. Sci., ser. 3, vol. 34, 1887, p. 193, pi. 1, 

 fig. 9; 10th Ann. Rep. U. S. Geol. Surv., 1890, p. 627, pi. 80, fig. 3. 



Original description. — "General outline of the valves subrotund, with 

 the exception of the nearly straight hinge line; anterior end slightly 

 narrower than the posterior; general surface rather strongly convex, 

 marked all around, except along the hinge line, by a strong marginal 

 groove within a rounded marginal rim; a single elongate protuberance 

 extends from the main body of the shell upward, just within the 

 anterior marginal groove and the liinge line, where it is most promi- 

 nent, and separated from the main body of the valve by a broad 

 sulcus extending from the liinge line down on the valve over two- 

 fifths the distance to the ventral margin. 



"The shell is thin and apparently very finely granulose [more likely 

 punctate]." 



Dimensions of the original type: Length 5.5 mm., height 4.0 mm., 

 depth (both valves) 1.9 mm. 



Remarks. — The specimen described by Walcott is the onlj^ one so 

 far seen of this interesting species. It retains both valves, which, 

 excepting that they are slightly displaced, are in good condition. 

 The test is thin, black, very slightly calcareous, and apparently 

 lamellose; hence liable to exfoliation. In composition and structure 

 it is essentially the same as the shell of A. troyensis, of Walcottella, 

 Dielymella, Beyrichona, and most of the other bivalved Crustacea 

 described in this work. For this and other reasons mentioned in the 

 remarks on the genus Aluta, A. rotundata and A. troyensis have been 

 removed from Aristozoe and Leperditia, to which they have been 

 previously referred. The close relations of A. rotundata to Beyrichona 

 also are discussed in the generic remarks. 



Viewed in its specific relations, A. rotundata is closely comparable 

 only with A. troyensis and A. primordialis, though readily distin- 

 guished from both. From the former it differs in many particulars, 

 the most striking and important of these being (1) the large depres- 

 sion in the antero-dorsal quarter, of which no sign is observable in 

 that species, and (2) its rounded rather than triangular shape. It is 

 nearer A. primordialis, but the outlines of the two forms are scarcely 

 less different, while the depressions and elevations in the anterior half • 

 are so obviously different in arrangement and degree of development 

 that it is unnecessary to point out the several disagreements. 



Occurrence. — Lower Cambrian; limestones interbedded in the shaly 

 slates on M. C. Tefft's farm, about 2 miles southeast of North Green- 

 wich, Washington County, N. Y. 



Holotype and plesiotype. — Cat. No. 17446, U.S.N.M. 



