part 4] THPJ EVOLUTION OF LIASSIC GASTROPODS. 343 



EXPLANATION OF PLATE XXII. 



Fig. 1. Zijgopleura cf. hlainvillei (Miinster). X 4. From the Lower Lias 

 {armatus zone), the brickworks, Folly Lane, Cheltenham. (L. Rich- 

 ardson Coll., L.a. 8.) See p. 326. 



2. Katosira trivia (Tate). Lectotype X 5. From the Lower Lias, 



Bridgend, Glamorgan. (Tate Coll., Museum of Practical Geology, 

 No. 7990.) See p. 335. 



3. Katosira {Rigauxia) noguesi (Dumortier). X 4. From the Lower 



Lias {armatus zone), Bince's Lodge, Wilton, Somerset. (J. W. 

 Tutcher ColL, No. 1038.) See p. 33 '^. 



4. Katosira concinna, sp. nov. Holotype X 4. From the Lower Lias 



{armatus zone), Clandown Quarry, Radstock, Somerset. (J. W. 

 Tutcher Coll., No. 1012.) See p. 334. 



5. Katosira cf. yo\ingi (Tate). X 5. From the Lower Lias {jamesoni 



zone) of the railway- cutting, Toddington, near Winchcombe, 

 Gloucestershire. (L. Richardson Coll., L.G. 28.) See p. 332. 

 G. Promathildiatenuicostata {'Poi'tlock). Embryonic whorls X 10. From 

 the Lower Lias of Ballintoy. (Tate Coll., Museum of Practical 

 Geology, No. 7965.) See p. 336. 



7. Katosira {Rigauxia) sp. X 6. From the Lower Lias {armatus zone), 



Clandown Quarry, Radstock, Somerset. (J. W. Tutcher Coll., 

 No. 1008.) See p. 336. 



8. ProceHthium ogerieni (Dumortier). X 8. From the Lower Lias of 



the tunnel-heaps. Old Dalby, Leicestershire. (Geological Collec- 

 tion, University College, Nottingham.) See p. 311. 



9. Paracerithium sp. X 2"5. From the Oolite Bed, falcifer zone. Upper 



Lias, Grantham, Lincolnshire. See p. 324. 



Discussion. 



Dr. F. A. Bather complimented the Authors on the great 

 interest of their work, and Dr. Trueman on his lucid presentation. 

 He would be glad to hear any explanation of the changes in 

 ornament. He assumed that the axial ridges were intensifications 

 of growth-lines, and that the spiral ridges were connected with 

 differential tension of the mantle. But were these changes due to 

 internal factors alone, or were they adaptations to some external 

 factors ? What could those factors be ? 



Dr. A. M. Davies congratulated the Authors on their in- 

 vestigation of a neglected group, and remarked on the general 

 rarity of fossil gastropods preserved perfect from protoconch to 

 m.outh-border. As to the two sets of ornament-lines, he suggested 

 that, while the axial or growth-lines were the expression of a time- 

 rhythm in the activity of the mantle-edge, the spiral lines (like 

 the radiating lines of lamellibranchs) were the expression of an 

 unequal local distribution of that activity, amounting in some 

 cases to a ' space-rhythm.' 



Mr. W. P. D. Stebbin^g expressed the opinion that the develop- 

 ment of ribs and ridges on the shells of gastropods was independent 

 of the strength of the shell, and that colour-markings developed in 

 an analogous manner. 



Miss McDonald emphasized the value of ontogenetic characters 



