PLEUEOTOJIID.E. 37 



aperture dilated in front. It may doubtless be referred to the 

 subgenus Leptoconns. From the Tertiary of Muddy Creek, Victoria. 



Fresented btj John Bennant, Enq, 



Family PLEUROTOMID^. 



In assigning the species here described to their systematic 

 positions in the Pleueotomid^, it may be remarked that in view of 

 the present unsatisfactory condition of the classification of many of 

 the so-called genera and subgenera in the family, some of the con- 

 clusions anived at must, of necessity, be of a tentative chai'acter. 

 The group has been divided into subfamilies according to the 

 presence or absence of an operculum, and the nature of that 

 when present. The systematic position of many of the species 

 is dependent on the situation of the sinus, its depth, and so forth. 

 The length of the anterior canal, details of ornament, peculiarities 

 of the radula, and the like, have also been permitted to rank high 

 — even as generic characters. 



The family as a whole is not of very great antiquity, its main 

 features having been carved out in the Eocene ; whilst many of 

 its broad characteristics were not evolved until the Miocene, or 

 early Pliocene. Possibly, no group of the moUusca has been 

 so prolific in yielding so many diversified forms in such a short 

 space of geological time. The variation of individuals in the 

 living as well as in the fossil state is often so wide as to render 

 it impossible to satisfactorily define the limits of range permissible 

 in species. That difficulty, combined with the latitude of the 

 personal equation, has led to the establishment of innumerable 

 so-called species of the Pleurotomid^, which appear to the writer 

 to be founded merely on ontogenetic characters. The problem, 

 already difficult with the living forms, is intensified when the 

 fossils are considered. Here we have no operculum to assist, 

 and many other points utilized in the classification of modern 

 Plkukotomid^ are missing. Shorn of the characters of their 

 opercula, the chief difference between Pleurotoma and Surcula, 

 for example, lies in the form and position of the sinus ; in the 

 former genus it is deeper, and is placed farther away from the 

 suture than in the latter. If these characters were in any way 

 constant they might be rendered useful ; but, as it is, we are 



