246 ABNOTTj OX CTCLOTELLA. 



jecting) disc, but the disc is marked by radiating dots or 

 lines. Smith, in his ' Synopsis/ had both in view : the first, or 

 true C. opercidata, he has described with sufficient precision, 

 although, in place of being concave or depressed in the centre, 

 as he says, I consider it to be convex or elevated ; the second 

 form is the one which he has figured in Tab. v, fig. 48, and is 

 also that which he distributed in his Lough Neagh slides; it 

 occurs near Ulverstone and Hull, and is probably not un- 

 common. 



Besides the above, Kiitzing has a species from the Lunne- 

 burg deposit, which he calls C. minutula; this occurs in many 

 deposits in this country. It is this which Smith obtained from 

 the Lough Mourne deposit, but which he has unfortunately 

 referred to C. antiqua, a species which does not occur in any 

 of the Irish deposits which I have examined. The largest and 

 finest specimens of it which I have seen are from the exten- 

 sive deposit near Toomebridge, between Lough Neagh and 

 Lough Beg, and that from Loch Leven, Kinross-shire, in 

 both of which it is mixed with C. Rotula, Sm. 



On carefully comparing the C. minutula from deposits with 

 the second form of C. operculata, I have little doubt of their 

 identity ; both have got the same kind of centre to the disc. 

 It seems to have a double coat of silex near the margin, or at 

 least two surfaces differently marked ; the upper one presents 

 a series of short, close, marginal striae, resembling a narrow, 

 striate, convex ring, surrounding the flat disc ; the under one 

 is flat, broad, and conspicuously striate from the margin to the 

 central portion or disc. Such are the appearances presented 

 w4ien both states are perfect ; but when the recent form 

 (usually confounded with ft opercu/ata) becomes abraded or 

 much macerated, it seems to pass into the other ; my impres- 

 sion, therefore, is that the one got in deposits (or ft minutula 

 true) assumes its distinctive appearance solely by long expo- 

 sure and maceration, and that it and the recent one (Smith's 

 tab. v, fig. 48) ought to be united. The name of minutula 

 is certainly objectionable, as the specimens, even when in 

 deposits, are often so large that they might be mistaken for 

 a small form of ft Rotula; but changes of specific names 

 lead to confusion when not transferred to another genus, and 

 it is therefore preferable to retain that given by Kiitzing. 



These five may be comparatively distinguished from each 

 other thus : 



1. C. Dallasiana ; ends of frustulc flat; centre of valve 

 bullate-rugose, marginal stria' coarse. 



2. ft Meneghimana ; ends of frustulc flat; centre of valve 

 neither striate nor bullate ; marginal strice coarse. 



