LIOCERAS CONCAVUM. 59 



We may notice the influence which the lateral process, whether pointed or rounded, 

 has had upon the ribs ; at first causing them to be more prominent in the middle of 

 the lateral area, and at a later stage making them curve forwards, and sometimes in 

 addition to thicken slightly ; at other times it has raised small waves along the middle 

 (a well-marked instance of this may be observed in PI. X, fig. 9) ; while again at 

 other times very small knobs are formed, as PI. VIII, fig. 7. (These knobs, 

 however, are inconspicuous, and specimens showing them are very rarely obtained.) 

 A noticeable feature in the termination of this species is that, when the test is 

 present, there is no indication of those constrictions which are otherwise visible ; 

 the test of the termination is, at the rim, as thin as paper; but is thickened 

 internally just behind, causing a marked constriction of the aperture. The same 

 thing may be noticed in Lioc. v-scriptum (PI. IX, figs. 1 and 3). 



The variability of this species in certain minute characters seems to be almost 

 endless. The differences which the various forms present are small, but fairly 

 persistent; and yet it requires a great amount of labour to separate all the 

 different varieties, and to trace them from youth to adolescence. I have had 

 depicted in the plates only the more striking forms, because it would have 

 been well-nigh impossible to thoroughly bring out in any drawing the various 

 differences which may be detected ; but I shall devote a separate article to 

 noticing, as far as possible, the many variations which the species exhibits. I 

 believe it necessary to fully work out and separate all forms of this or other species 

 which differ from the type ; and I scarcely agree with the common practice of 

 placing several various forms under one specific name, in collections or otherwise, 

 without note or comment as to their differences. The reason for this opinion is 

 that I think a thorough investigation of such forms is the one way to arrive at a 

 true idea regarding the descent of certain species from others ; because a very 

 slight variation may give us the necessary clue, or the form may have developed in 

 an unexpected manner. For such critical work as this the figures given by most of 

 our older authors were not sufficiently exact ; and therefore their specimens should, 

 when possible, be redrawn. As regards this species, I have had the opportunity 

 of thoroughly examining several hundred specimens ; and in fact it is, or rather 

 was, so plentiful at Bradford Abbas as to be frequently neglected by collectors. 

 To this neglect, in all probability, the scarcity, in most collections, of certain varieties 

 and similar-shaped species is partly due, because, as their differences could not 

 be noticed when they were embedded in the matrix, the specimens would be left 

 alone, under the impression that they were nothing more than the common Lioc. 

 concavum. 



When we consider the abundance in which specimens of this species occur, we 

 can scarcely marvel at the variability which they exhibit ; and it is on account of 

 this variability no easy matter to point out all the differences between this and 



