220 ON THE TOPOGRAPHY AND GEOLOGY ' 



M . r u (lis. GaM), ii. s. 



Shell short, broad robust, sides curved ; spire about equal to the mouth ; upper whorls nearly flat on the side ; 

 suture bordered by a sliglitly thickened margin ; surface marked by a few large rounded revolving lines crossed by 

 irregular lines of growth, so as to produce a semi-cancellate appearance. Aperture broad, not contracted in advance ; 

 iimer lip with four folds of which the most posterior is very prominent and the anterior is very indistinct ; outer lip 

 serrate, the notches corresponeling to the superficial ribs. Length lA inch, width .6 inch. 



The short, thick form and I'udely cancellate surface sufficiently distinguish this 

 rare shell. •, - , 



M . titan. Gabb, n. s. 



Shell very large, elongate spire a little shorter than the aperture ; whorls about ten or eleven (apex broken), 

 Slightly convex on the sides, suture linear or bordered by a very faint thickening of the margin of the succeeding 

 whorl. Surface nearly smooth, marked only by a few faint revolving impressed lines in the adult. In the young 

 shell these are more marked, and are broken by the traces of the lines of growth whicli are hardly perceptible 

 except in the depressions. Anteriorly there are sometimes traces of indistinct revolving ribs. Aperture long and 

 narrow, slightly narrowed in advance ; colvimella encrusted, esiaecially in front, where it is gently twisted, carrying 

 one very faint and four distinct folds. Outer lip thick and rounded on the margin, not striate. Length 6 inches, 

 width 1.75 inch. 



This shell, which rivals M. episco2)aUs in size, resembles in its younger stages 

 3f. Isabella in form, but can be distinguished by its smooth surface, which also 

 sej^arates it from all the other fossil Mitras of Santo Domingo. \. 



Not unlike 3f. scrohiculata, Brocc, of the jS'. Italian miocene, but proportionately 

 more convex, with a larger body whorl and with the columellar folds more transverse. 



M. symmetrica. Gabb, n. s. 



Shell slender fusiform ; spire and mouth about equal ; whorls ten, slightly convex on the sides, suture, distinct. 

 Surface marked by numerous, closely placed, flattened revolving ribs, the interspaces crossed by prominent lines of 

 growth which do not appear on the tops of the ribs. Ajierture moderate, narrowed sliglitly in front. Columella 

 with four well marked plaits. 



From 31. Henekeni and 3L longa this can be readily distinguished by its flat ribs ; 

 from the first by its more numerous columellar folds and from the second by its more 

 rounded foi-m. It is not impossible, however, that it may prove to be the young of 3L 

 titan., from which it differs in having a straight canal, and in its ribs ; two characters 

 whicli are not ahvays unalterable as shells 2"row older. 



M . t o r t u o s a . Gabb, n. s. 



Shell elongate, sub-tiirretted ; spire about as long as the aperture ; wliorls flat on the sides, convexly truncated 

 adjoining the suture ; body whorl narrowed iu front and produced into a tortuous canal. Surface marked by about a 

 dozen longitudinal ribs, acute on their summits and with concave interspaces. These ribs begin at the suture, and 

 on tlie body whorl cover all of its convex portion. Between these are revolving* impressed lines of variable 

 intensity, which never cross the summits of tlie ribs. Beyond the termination of the longitudinal ribs, and espe- 

 cially on the canal, these lines are stronger. Aperture long and narrow, contracted into the canal in advance ; colu- 

 mella twisted, bearing four prominent folds. Outer lip faintly notched near the suture. Length 1.3 inch, width 

 .45 inch. 



This species belongs to the group of Costellaria, Sw,, but the contraction into an 

 anterior canal is more marked than in any other species known to me. It is nearest 

 in style to 31. seinr/'asciata, Lam. 



