M0LLUSCA OF THE ' CHALLENGER ' EXPEDITION". 453 



stronger at the extreme point. Colour alabaster-white, almost 

 translucent from the thinness of the shell. Spire conical, with 

 profile-lines a little interrupted by the contraction of the sutures. 

 Apex slightly eroded, but evidently small and rounded ; the em- 

 bryonic whorls seem to be about two. Whorls 8 in all, of rapid 

 increase, but that rather in breadth than height ; the last is 

 extremely large and tumid ; they have a long and gently sloping 

 shoulder above, and are cylindrical below the blunt angulation 

 which bisects them ; the last contracts slowly from the angle, and 

 is tumidly convex on the base, the curve of which sweeps on, 

 on the right side, uninterruptedly to the point, while on the 

 left side it passes by a rather deep concave curve into the lop- 

 sided, triangular-shaped, small-pointed snout, which, however, 

 projects very much in the line of the axis. Suture strong, a little 

 channelled from the overlap of the calcareous surface of the 

 inferior whorl, well defined from the angulation made by the 

 meeting of the whorls. Mouth large, long, angularly pear- 

 shaped, and oblique to the axis. Outer lip very thin ; it is 

 a high arched curve, straight and steep on the shoulder, and 

 regular from the angulation downwards : the edge retreats to 

 the left on leaving the body, and forms a wide, deep, ellip- 

 tically rounded sinus occupying the whole shoulder, having 

 above it a short triangular shelf, and below, the high elbow 

 formed here by the prominent sweep of the lip-curve, which 

 does not retreat till near the end of the snout. Inner lip is shal- 

 lowly excavated in the substance of the shell, which rises beyond 

 it with a slight edge ; it is broad, and winds round the pillar ; the 

 line of it is slightly convex on the body and concave at the 

 junction with the pillar, which is short and conical, obliquely cut 

 off to a point, with a long, fine, rounded, and slightly twisted 

 edge. H. 095. B. 057. Penultimate whorl, height 0'15. 

 Mouth, height 0*62, breadth 0*35. 



This is a very peculiar and beautiful species. It unites some- 

 what of the form of Clavatula with the delicate texture and sculp- 

 ture of Drillia. The animal is so much unlike other Pleurotomas 

 as to make the classification of the species very doubtful. Thesbia, 

 as indicated by Dr. Grwyn Jeffreys and defined by Gr. O. Sars, 

 may receive it in the meantime. 



50. Pleurotoma (Thesbia) pruina. n. sp. 

 St. 78. July 10, 1873. Lat. 37° 26' JSL, long. 25° 13' W. San 

 Miguel, Azores. 1000 fins. Globigerina-oozt. 

 linn, journ. — zoology, vol. xv. 35 



