356 GASTEROPODA OF THE INFERIOR OOLITE. 



N.B. — The figured specimen does not exhibit these latter features distinctly. 



Relations and Distribution. — This very commonplace little shell seems to differ, 

 as far as I can judge, from any species in the Inferior Oolite, though it may 

 resemble some of the smooth Turbos recognised in the Lias. Notwithstanding 

 the absence of any well-defined denticle, the character of the aperture rather 

 reminds one of Ata/phrus. 



Not very uncommon in the " Base-bed" at Lincoln. 



Group II. — Sculptured Shells. 



One of the most difficult problems which present themselves for solution in 

 Jurassic palaeontology is the correct generic classification of the numerous members 

 of the Turbinidas coming under Group II. Except as regards the genus Trochus, 

 which is undoubtedly represented in considerable numbers, the identification of the 

 modern genera, Monodonta, Turbo, Delphinula, &c, is to a certain extent empirical. 

 I have always had an objection to the acceptance of Monodonta, but there is safety 

 in Turbo as a general term, which may be held to include various sections by some 

 recognised as distinct genera. Delphinula, too, has had its advocates, and many 

 species of Jurassic Turbinidae have from time to time been thus classified. If, 

 then, I continue to use the genus Monodonta for Jurassic shells, it is only in a 

 conventional sense, as Turbo would do just as well. For the same reason no 

 diagnosis is offered either of Monodonta, Turbo, or Delphinula. 



291. Monodonta pisolitica, sp. nov. Plate XXIX, fig. 17 and ?fig. 18. 



Description : 



Height . . . . .4 mm. 



Width . . . . .3-5 mm. 



Spiral angle ..... 65°. 

 Shell trochiform, thick, imperforate; apex rather obtuse, spiral angle regular. 

 Number of whorls about five ; the apical ones are smooth and round, the remainder 

 flat, and ornamented by four tuberculated spirals, which in the body-whorl present 

 some differences, the anterior spiral at the angle of the shell having larger 

 granulations than the others. Body-whorl subangular and rather more than half 

 the total height ; base full, and ornamented with granulated spirals rather finer 

 than those at the sides. Outer lip circular; columella nearly straight, andreflexed 



