MARSHALL : ADDITIONS TO " BRITISH CONCHOLOGY. l> 171 



of Sowerby's "Index." Then the nomenclature of the genus shows 

 a great poverty of resource and want of intellectual ingenuity ; not 

 only species, but their varieties are called alba, albella, pallida, lactea, 

 etc,, names that are equally applicable to almost every species and 

 variety in the genus. 



The one good and permanent character of the Odostomice is un- 

 questionably the sinistral and inverted apex, which is constant 

 throughout the genus, although in some littoral species, and in those 

 frequenting rough ground, this may not be observable on account of 

 the apical whorls being often ground down to an obtuse point. The 

 character of the tooth is unreliable, as is also that of the umbilicus, 

 both of which are calculated to mislead beginners ; the former is 

 often obscure or altogether absent in species characterised by a 

 tooth, while the latter varies considerably, even in the same species 

 when taken from one locality, from being open in some examples 

 to a mere fissure or imperforate in others. The following notes 

 indicate great variation in this character. Many of the Odostomice, 

 when aged, develope a deeper umbilicus and a more or less complete 

 peristome of the aperture, as in some species of Rissoce. The 

 umbilicus, where present, is just behind the tooth, and suggests a 

 needle having pierced the columella and made a slight excrescence 

 on the other side. Most of the species in the Turbonilla and Eirii- 

 mella sections have the lower whorls less compact, with deeper sutural 

 lines, as in Turritella terebra. 



Dr. Jeffreys has remarked on the difficulty of obtaining a perfect 

 pair of Cardium edule, but it is equally difficult in nearly all species, 

 to those who possess the critical faculty, to get two specimens that 

 shall be perfect facsimiles, just as it is said there are no two human 

 beings alike, or no two leaves on a tree. The forms of the Odos- 

 tomice are so varied that it would not be difficult, with large series of 

 examples, to run one into another in regular gradation ; but an accus- 

 tomed eye soon learns to detect, as by instinct, the family likeness of 

 each species. In the sculptured ones there is of course no diffi- 

 culty, and the rigid observance of one form as the type is not abso- 

 lutely necessary ; but in the smooth species, determinable as they are 

 principally by shape and outline, and all of them more or less minute, 

 it is essential to have a fixed type and to adhere to it. And if the 

 species themselves are difficult to identify, the figures even in the 

 standard works do not help the student much, from the difficulty of 

 determining what are the type forms. Three figures of the same 

 species in three different works may be found to be quite unlike each 

 other, though no doubt the authors had the same species before them. 



[To be continued). 



