Such a slide may be numbered, named, examined 

 and mapped as below : — 



The arc — . represents the base, and the cross lines 

 the transverse bars of the jaws. By this method any 

 number of jaws can be diagrammatically presented, 

 and by thickening, thinning, or lengthening the 

 strokes, the variations we find on the real specimens 

 may be represented. It follows, that if in such a 

 tabulation it had been found that II. ncmoralis, hor- 

 tensis or hybrida had always a certain proportion of 

 cross-bars, or that these ribs were differently placed, 

 a classification might have been built up on these 

 characters ; but although I long ago learned that the 

 ncmoralis had larger jaws, and was generally better 

 ribbed, yet the number of bars varies so much that 

 specimens with two bars occur, and rarely we find 

 five bars in hortensis and hybrida. 



The examination of one hundred and forty-two 

 jaws gives the following results : — 



A glance at this table shows that II. kortensishas the 

 lowest average of bars 2'8, hybrida 3'3, and nemwralis 

 4*4. A reference to my remarks on the shells, shows 

 that this order is borne out in them too, hortensis 

 thin, hybrida stronger, and ncmoralis a solid and 

 more compact shell than either. 



In a district near Leeds, out of twenty-eight speci- 

 mens (see table below), I find the proportion slightly 

 increased, ncmoralis averaging five bars per jaw, and 

 hortensis barely three. Of hybrida I only took two 

 specimens. 



An opinion which I have formed after much reflec- 

 tion on this jaw variation from tables and observa" 



tions may be presented in a few words. The ribs 



being auxiliaries of the odontophore, and abounding 



especially in terrestrial forms, 



vary in different situations to 



suit food plants. 



In a set made up of speci- 

 mens collected by myself at 

 Driffield, and from its neigh- 

 bourhood by kind donors, I 

 find the ncmoralis have generally 

 two to four, and rarely five 

 cross-bars, and the hortensis the 

 same. These specimens were not near roads, but 

 under walls and along wood sides. Specimens of 

 ncmoralis which I gathered on the sea cliffs at 

 Whitby, very exposed situation, herbage rank, hard 

 and dry, had all, with one exception, a four-ribbed 

 specimen, five or six bars to each jaw. 



The ncmoralis gathered on a road-side in Leeds, 

 Carboniferous formation, are very strong and ha\ e 

 dense jaws. Twenty-three full-grown specimens 

 yield on examination ten with 4, ten with 5, two 

 with 6, and one with 7 bars. I have two other 

 examples of jaws with 7 ribs, from two different 

 localities and limestone formation, but each were 

 from hard herbage and dusty road-sides. Four 

 quarter-grown specimens from a road-side in Leeds 

 give one 4, one 5, and two 6 ribbed-jaws, and three 

 half-grown ones, two 4, and one 6 barred jaw. 



In four 5 banded ncmoralis taken in a damp situa- 

 tion near Tadcaster, near to which I have procured 

 hortensis, hybrida and H. arbitstornm, I find two 

 with 2, one with 3, and one with 4 ribs, which are 

 particularly acute and narrow ; two hundred yards 

 away on the dry dusty road-side, the ncmoralis yield 

 a high average of jaw-ribs, one having 7 ribs. A 

 slide from the river-side, and this locality compare 

 thus : — ■ 



The specimens taken where the herbage is strong 

 and hard, yield jaws which are deeper in colour. The 

 largest specimens from each of the slides last men- 

 tioned give for the river-side a jaw T oo in. long (across), 

 and T §5 in. broad in the centre, the road-side one being 

 too x Ti>o m< The most distortious of jaws are in hor- 

 tensis and hybrida, from road-sides, probably due to 

 softer jaws and harder foods. 



In the article on H. arbustorum mentioned above, 

 the writer says, when speaking of jaws, "In old 

 specimens there are sometimes six ribs." Here is 

 inferred that age adds the ribs ; if this were so my idea, 



