MUSEUM OF COMPAKATIVE ZOOLOGY. 2G7 



Mathilda Rushii Dall. 



Shell white, sevcn-whorlud, with the transverse sculpture Btronger and 

 sharper than the sjiirals; consisting of elevated thickish sharp-edged lanielliJO 

 like those of some Scalce, the edges of which rise above the whorl, and cross 

 it from the suture to the margin of the basal disk, the surface of which is 

 nearly smooth. The whorls are somewhat shouldered by an obscure spiral, in 

 front of which are three others; the space between the first and second is 

 slightly greater than that which separates the others. The spirals are indicated 

 rather by nodulation of the edges of the lamellse than by any positively raised 

 thread under them, except on the last whorl. Columella thin, arched, mouth 

 rounded, base flattened on the disk, imperforate, nucleus small, lucid. Max, 

 Ion. of shell, 5.0; max. lat. of shell, 2.0 mm. 



Habitat. Bed of the Gulf Stream, in 351-465 fms., Florida Straits, Dr. 

 W. H. Rush, U. S. N. Also off Fernandina, in 294 fms., sand, temperature 

 4C0.3, U. S. Fish Commission. 



This pretty little species has a particularly sharp-edged surface, recalling the 

 laminae on the surface of a file. It looks very much like an Acrilla, except 

 for the nucleus and the subangular aperture. Some specimens show one or 

 two old apertures, like varices, projecting above the other sculpture. 



Mathilda scitula Dall. 



Shell light brown, eight-whorled, beside the nucleus, which is large, helicoid, 

 and of two or three whorls; including the carina of the basal disk there are 

 five spirals; counting forward from the suture on the last whorl. No. 1 is of 

 moderate size and nodulous; No. 2, very fine and undulated; No. 3, strongest 

 of all, and evenly nodulous; Nos. 4 and 5 are smaller, subequal, and nearer 

 each other than No. 4 is to No. 3 ; the basal disk is flattened-concave, with 

 curved radiating incremental lines, and fine spiral threads, of which one near 

 the pillar and one near the carina are larger than the others; the carina is 

 buried in the suture as the shell grows, above it the surface of the whorls 

 under the spirals is crossed by numerous strong narrow elevated lamellse, the 

 interstices of the reticulation thus formed being deep and squarely cut. The 

 aperture is nearly square, except as modified by the sculpture; the peristome 

 is thin, the pillar thin, simple, and somewhat reflected. Max. Ion. of shell, 

 5.25; of last whorl, 1.8; max. lat. of shell, 1.7 mm. 



Habitat. Off Cape Hatteras, in 49-63 fms., sand, at U. S. Fish Commission 

 Stations 2595 and 2596, bottom temperature 75°. F. 



This is more slender and elongated than either of the other species, and its 

 sculpture is different. It has a larger and more globular nucleus than M. bar- 

 badense or M. yucatecana, though both of these are larger shells. It is per- 

 haps most nearly related to 31. sinensis Fischer, which it resembles in general 

 form though not in the minor details of sculpture. In M. scitula, sculpture 

 aside, the whorls are rounded and the sutures distinct; there is no trace of any 

 umbilicus. 



