98 DENTALIUM-LiEVIDENTALIUM. 



M. Cossmann founded this group to include smooth species with 

 the shell oval in section, the posterior orifice without a slit. In deal- 

 ing with recent species we find it practically impossible to draw the 

 line between circular and slightly oval forms; and the apical char- 

 acters seem insufficient ground, in our opinion, for the separation of 

 species with no slit from those with an Antalis-like extremity. In 

 accepting the distinguished French palaeontologist's group, we there- 

 fore enlarge its bounds beyond those originally intended ; and are 

 alone responsible if future investigators find that heterogeneous mate- 

 rials are included therein. 



I. Apex simple, without notch or slit, Group of D. laeteum, p. 98. 



II. Apex with a slit or notch. Group of D. matara, p. 102. 



Group of D. ladeum. 



Smooth, polished, moderately curved shells with the apical orifice 

 simple, not notched or slit. 



Of recent species, these are nearest to the type of Lcevidentalium ; 

 but the distinction between a notched and simple anal orifice is in 

 actual practice rather delusive. One encounters many apparently 

 perfect individuals of species typically notched, in which the apical 

 margin is entire ; and yet no fracture other than the normal trunca- 

 tion of the apex with increasing age, may appear. It is only when 

 numerous specimens of various age are available for study, that the 

 systematic position of some species can be ascertained. Occasional 

 old specimens of the group of D. semistriatum have the characters of 

 the present group ; and more frequently specimens of the D. matara 

 group may be looked for here. The key given below must therefore 

 be used with caution. It is moreover rather unsatisfactory on 

 account of the absence of readily describable characters in these 

 smooth simple shells. 



Key to Species. 



(Consult also the group following this, p. 102, and that preceding, 

 p. 85). 



I. Large and stout, length 75 mill., 7 J times the diameter ; rather 

 straight, polished, opaque and solid, rapidly increasing. Cape 

 Horn, lebruni, p. 102. 



II. Small, white, nearly straight ; length 15 mill., nearly 9 times 

 the diameter. New Zealand, ecostatum, p. 102. 



III. Much narrower, the diameter contained over 10 times in the 

 length. 



