VESTINAUTILUS SEMIPLICATUS. 91 



may have been passed over or mistaken for V. panc'icavinatus, witli which it is 

 found associated. 



Localit)/. — Limerick (probably near the city). 



Vestinautilus SEMIPLICATUS, sp. nov. Plate XXV, figs. 1 a, b, 2 a, h. 



Description. — Shell thick-discoid, of a robust habit, consisting of about four 

 volutions, all exposed in a deep umbilical cavity having very steep sides. The 

 whorls increase rather slowly in diameter, and their section is roughly decagonal ; 

 the peripheral area being more raised in the centre than at the sides, thus forming 

 an irregular arcli. The edge of the umbilicus is angular in the specimen before 

 me (the only one known to me), and there is no rim or keel properly so called ; a 

 prominent one, however, is present within the umbilicus at a very slightly lower 

 level than the margin of the latter, and hence forming a sort of platform, from the 

 inner edge of which there is a steep slope down to the suture of the preceding whorl. 

 Beginning at about the last half of the last volution the angularity of the whorl 

 disappears, and at the same time the keel at the umbilical shoulders becomes 

 plain and rounded. Just at the commencement of the body-chamber, on either 

 side two large folds make their appearance ; these are succeeded by another 

 rather stronger pair, and these again (on the cast) by a pair of prominent 

 tubercular folds which formed the outer boundaries of a former aperture, the 

 hyponomic sinus of which is indistinctly seen on the cast in continuation of these 

 folds. Two other pairs of prominent tubercular folds occur, the last of Avhich 

 formed the edge of the present aperture of the shell, the margin of which can be 

 traced in the cast with its broad and deep hyponomic sinus. The body-chamber 

 does not exceed half the length of the last volution. The septa and siphuncle are 

 not known, as the test completely covers the septate part of the shell. There are 

 no surface ornaments, but lines of growth which mark the form of the hyponomic 

 sinus, already described, are seen at different places on the peripheral area ; and, 

 conforming* in direction with these lines, there are also coarse and inconspicuous 

 folds. Two faint ridges appear on each side of the peripheral area at its 

 narrowest part ; these were probably stronger in the young shell. 



Dimensions. 



Diameter of shell 



„ umbilicus (from edge to edge) 



,, „ (from suture to suture) 



Height of outer whorl (dorso-ventral) 



Thickness at umbilical margin 



Width of periphery of outer whorl . 



Unique specimen, from 

 Ratlikcale. 



115 mm. 



80 „ 

 45 „ 

 47 „ 

 70 „ 

 67 ,, 



