266 BULLETIN OF THE 



Family MATHILDIIDiE. 

 Genus MATHILDA Semper. 



In form these shells recall Bittium and Turrilella ; they have a blunt apex 

 upon which the little heterostrophe yl^eoriis-shaped nucleus is set on its edge, 

 and often on one side of the axis. The operculum is said to be nmlti.spiral and 

 externally concave, the soft parts are externally not unlike those of Turrilella, 

 the dentition is unknown. 



Tliis group is represented abundantly in the Eocene Tertiary of the Paris 

 Basin. There are a few recent species widely distributed, of which two or 

 three are Mediterranean, and one has been described from the West Indies, by 

 Morch, under the name of M. trochlea. This is bicarinate, and measures only 

 2.0 mm. long by 0.5 mm. wide. 



In the subgenus Gegania Jeffreys the shell is more trochoid in form, and the 

 heterostrophe nucleus is sunken, showing only a small part of one of its whorls. 

 His supposition that it is dextral is erroneous, as his typical specimens show 

 plainly. 



Mathilda yucatecana Dall. 



Plate XX. Fig. 7. 



Bittium 1 yucatecanum Dall, Bull. M. C. Z., IX. p. 90, 1881. 



Habitat. Yucatan Strait, 640 fms. Also at U. S. Fish Commission Stations 

 2415, in 440 fms., sand, off the coast of Georgia, bottom temperature 45°. 6 ; 

 and 2668, in 294 fms., gray sand, off Fernandina, Florida, temperature 46°. 2 F. 



Nearly all the species of this genus have a patulous extension of the anterior 

 part of the peristome resembling the obsolete notch of Bittium. The absence 

 of varices, and, when not decollate, the heterostrophe nucleus asymmetrically 

 set on edge, are distinctive characters. There are some features strongly sug- 

 gesting relations with Triptychus. 



Mathilda (elegantissima var. ?) barbadense Dall. 



Plate XXVI. Fig. 10. 



Shell resembling M. elegantissima Costa, but proportionally more elongated, 

 with a nucleus only about one fourth as large as that of the Mediterranean 

 species; with the peripheral spiral so large and sharp compared with the others 

 as to carinate the whorls; and with the disk of the base covered with small 

 spirals and proportionally smaller. Shell brownish with seven whorls, exclu- 

 sive of the nucleus, the anterior edge of the peristome somewhat produced, 

 columella simple without any chink behind it. Max. Ion. of shell, 6.2; of 

 last whorl, 2.7 ; max. lat. of shell, 2.5 mm. 



Habitat. Barbados, 100 fms. 



This is perhaps a variety of elegantissima, but I doubt their identity. 



