4 ANNALS NEW YORK ACADEMY OF SCIENCES 



of Allerisma costatum and a renewed consideration of its characters, I 

 believe that my original estimate of its relationship to Pleurophorella 

 papillosa was erroneous. The strong concentric plications which stop 

 abruptly at the umbonal ridge indicate a different type of shell. It is 

 somewhat doubtful whether a papillose surface is a real character of A. 

 costata, which is apparently a much flatter shell, with thin test and pos- 

 sibly different structure in the lunule and escutcheon. It clearly does not 

 belong with Alula, however, but has all the superficial characters of 

 typical Sanguinolites. 



Alula squamulifera 



Shell rather small, very transverse. Width about 3.5 times the greatest 

 height. Greatest height near the anterior end at the umbo, which is situated 

 about one-sixth of the entire width back from the anterior margin. Ventral 

 border gently convex in the anterior half, nearly straight or faintly concave 

 posteriorly. Dorsal outline gently concave or nearly straight, contracting pos- 

 teriorly with the ventral. Posterior outline oblique and more or less sharply 

 truncate. Anterior outline straight above, strongly rounding below. Convexity 

 usually rather high, though variable, sometimes rather tumid in the umbonal 

 region. Beaks large, prominent and incurved, situated relatively close to the 

 anterior extremity. Umbonal ridge prominent, usually strongly angular toward 

 the posterior end, more obscure in the umbonal region. The post-cardinal 

 slope is divided by a second plication about intermediate between the umbonal 

 ridge and the cardinal line, above which the narrow strip of shell is nearly 

 horizontal. Surface marked by fine, radiating ribs which are confined to the 

 portion of the shell below and in front of the umbonal ridge. This k^.^tme 

 might better be described as made by narrow stria 3 , the elevations between 

 which are covered with closely arranged, fine, flat scales, which recur at equal 

 intervals on adjacent ribs and have also the appearance of interrupted con- 

 centric lamellae. The ribs are more than radiating rows of scales, since the 

 spaces between them are depressed. The scales are sometimes more or less 

 curved with the convex side uppermost, especially at the anterior end, where 

 they are replaced by two or more rows (the radiating arrangement often not 

 being apparent) of minute spines or papillae. Apparently, these spines become 

 more or less compressed toward the middle of the shell and then coalesce at 

 their edges. If they are not quite in alignment, the curved appearance noted 

 above results. The post-cardinal slope, which, as already mentioned, lacks 

 radiating ribs, is nevertheless marked by these flattened scales, which tend to 

 be arranged in concentric rows without, however, becoming connected into 

 continuous lamella?. No radial arrangement is here apparent. 



The internal structures are imperfectly known. The right valve bears two 

 linear teeth, one before and one behind the beaks. The posterior tooth is long, 

 about two-thirds the entire length back of the beaks. The anterior tooth is 

 much shorter, about one-half the length of the anterior outline. Whether a 

 small cardinal tooth was developed between these at the umbo is not clearly 

 shown, but such a structure is indicated. In the left valve, there appear to be 

 linear sockets coiTesponding to the teeth of the right. A large anterior scar is 

 Indicated. 



