SCIENCE-GOSSIP. 



301 



the fourth still smaller and still further distant ; 

 the anterior portion of the second series possesses, 

 in addition to the oblique curved denticle, a slight, 

 straight, horizontal denticle. Another specimen, 

 also in my collection, measuring 5-5 millimetres in 

 diameter, has three horizontal denticles in the first 

 series, while the second series is similar to that in 

 my other two specimens. It possesses, however, in 

 addition, one posterior and two anterior denticles 

 of a previous set, separated from the mature set by 

 a distance of i millimetre. 



Plectopylis rctifera (figs. 43«-c), from South India, 

 was described by Dr. Pfeiffer in the " Proceedings 



Fig. n3.— Plectopylis rctifera. 



of the Zoological Society," 1845, p. 73, and figured 

 in Reeve's " Conchologia Iconica," t. 173, f. 1170 

 (1853), and in Hanley and Theobald's "Conchologia 

 Indica," t. 87, ff. 8 and 9 (1872). As the armature 

 has never been figured, I am glad to have an 

 opportunity of doing so. The shell is convexly 

 conical, narrowly umbilicated and acutely keeled ; 

 it is dark corneous, translucent, finely and 

 regularly plicated by raised ribs above, finely and 

 closely ribbed below. The periphery has an acute, 

 compressed keel, above which revolve two raised 

 spiral ridges, which can be traced to the em- 

 bryonal whorl, the lower one being provided with a 

 fringe of coarse hairs. The shell is composed of 

 6^ slowly increasing convex whorls, while the base 

 is flattened and shining, a little tumid round the 

 umbilicus, which is deep and narrow, suddenly 

 widening at the last whorl. The aperture is sub- 

 quadrate and elongated ; the peristome is simple, 

 acute, scarcely reflected below. The parietal 

 armature consists of a single, strong, vertical plate, 

 slightly sinuate, but not notched, giving off a slight 

 support anteriorly a little below the upper ex- 

 tremity (see fig. 435, which shows both the parietal 

 and palatal armatures from the posterior side). 

 The palatal armature is distinctly visible through 

 the shell-wall, and consists of two series of 

 denticles, the upper series is composed of : pos- 

 teriorly, a strong, short, vertical, flattened tooth, 

 and, anteriorly, a minute, horizontally elongated 

 denticle, in a line with the base of the posterior 

 tooth ; the lower series is composed of : 



posteriorly, a smaller, flattened, vertical tooth, and, 

 anteriorly, in a line with its top, a minute, horizon- 

 tally elongated denticle, and, in a line with its base, 

 a larger denticle, elongated obliquely. Above the 

 periphery occurs, in addition, a small, horizontal 

 denticle, and below the umbilical angulation a 

 short horizontal fold. The two specimens figured 

 are in Mr. Ponsonby's collection, and measure 6 

 millimetres in diameter. The one shown in fig. 43c 

 is not quite mature, the newly-formed palatal 

 armature, near the aperture, consisting of only one 

 horizontal and two vertical denticles. Colonel 

 Beddome has obligingly allowed me to inspect a 

 large series of specimens of this species from the 

 Tinnevelly Hills ; of these, nine full-grown speci- 

 mens possess only one set of denticles ; five not 

 quite full-grown specimens possess two sets of 

 denticles each, the older (immature) sets being 

 complete, while the newly-formed sets consist of 

 one, two, or three denticles ; four immature speci- 

 mens have only one set of denticles ; ten immature 

 specimens possess two sets of denticles. Of the ten 

 specimens last mentioned, three have the older set 

 complete and the newer set partly formed, five 

 have the older set incomplete (partly absorbed) 

 and the newer set complete, while, finally, the 

 two remaining specimens have both sets complete. 

 It may, therefore, safely be inferred that the 

 older set does not become absorbed until the new 

 set is completed. In a few instances I have 

 observed that the two lower anterior denticles 

 have become fused. 



(To be continued.) 



