ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 33 



namely, the surface is always distinctly and closely pitted. In Indi- 

 ana, on the contrary, the surface is always at least smooth and gen- 

 erally polished, the punctae, which are usually present, being very 

 small and distant. 



The relations of Walcottella to Aluta Matthew, as redefined by the 

 writers, are probably much less than may appear on first thought. 

 Aluta often has a small node in the anterior half of the valves that 

 may suggest the median tubercle of a Walcottella like W. limatula. 

 On further study the node in the former is recognized as representing 

 the "ocular node" and not the "muscle spot." The test of Aluta 

 also is quite different, being thinner, smooth, and almost membranous. 

 Finally, the general shape is different, the valves having a more 

 decided retral swing and the antero-cardinal angle being sharper 

 and the most prominent part of the anterior margin. In fact, Aluta 

 is believed to represent another family and to be much more closely 

 allied to the new genus Dielymella than to Walcottella. 



WALCOTTELLA APICALIS, new species 



Plate 4, Figures 1, 2 



Description. — Carapace subovate, slightly oblique, widest posteri- 

 orly; length and height respectively about as 4 is to 3 or 5 to 4; 

 hinge straight, rather long, about three-fourths of the total length; 

 antero-cardinal angle sharp, 100° to 110°; postcardinal angle distinct 

 but much wider, about 130°; excepting the cardinal edge and small 

 parts just beneath its extremities, the outline forms a nearly regular 

 elliptic curve. Surface of valves depressed conical, sloping at first 

 sharply and then more gradually, though not uniformly, in all direc- 

 tions from the apex of the strong, centrally situated conical eminence; 

 slope to the cardinal edge more concave than to the ventral margin. 

 A wide, flattened border, very distinct on the right valve but scarcely 

 defined on the left, extends around the free edges. Two low ridges 

 usually distinguishable in the space between the median tubercle and 

 the antero-cardinal angle. The anterior of these two ridges is the 

 longer and may be recognizable halfway down the anterior side. The 

 inner swelling, which probably embraces the "eye spot," is flanked 

 on either side by an ill-defined depression. Surface of shell closely 

 punctate, or rather reticulate, the pattern, as to size and arrangement 

 of punctae about as in W. hreviuscula. (See pi. 4, fig. 5.) A tendency 

 to concentric arrangement of the punctae is occasionally observable 

 in the middle part of svirface. 



Dimensions of an average example : Length 5 mm. ; greatest height, 



4 mm. ; thickness between apices of median nodes estimated at nearly 



5 mm. 



2607—31 3 



