part 4] JURASSIC CHROIv'OLOGT : LIAS. 297 



D e s c r i 2) t i n . — Serpenticone ; platy subpachygyral, latumbili- 

 cate to j^erlatumbilicate, carinati-sulcate, costate, gammiradiate. 



Eemarks. — This inflated Arietites is sufficiently well repre- 

 sented in Wright's figure and description, although Wright's form 

 attained to soiiie\Yhat greater stoutness at an earlier age. Keynes's 

 shell has similar proportions to Wright's 1S79 figure, but is some- 

 what .different in ribbing.^ The shell now described is neither 

 A. turneri nor A. hroohi, as the proportions stated on p. 298 will 

 show ; it is an inflated development of A. turneri, and is not 

 intermediate between A. turneri and A. hroohi, as the latter 

 belongs to the compressed stock. 



Apart from its geological interest, which will be discussed pre- 

 sently (p. 311), and its biological interest as an inflated Arietites^ 

 the shell now described is noticeable for a pathological development. 

 On the periphery of the last quarter of the outer whorl the strong- 

 keel and furrows suddenly cease, and a rounded venter is produced : 

 the Inuch ventrallj^-projected radii which accompanied keel and 

 furrows — one of the generic characters of Arietites — also suddenly 

 give place to radii crossing the venter with a slight linguiform bend 

 (PI. XXXI, fig. 5). This change is presumably due to injury, and 

 the injmy has brought on highly-accelerated catagenesis of the 

 periphery, but has not affected the strength of the lateral ribs 

 We know that catagenesis is the reversal of anagenesis, and we 

 know from allied stocks that keel and furrows ultimately die away 

 leaving a rounded venter ; also that highly-projected ventral radii 

 will return to a less projected state. But such catagenesis is 

 usually gradual, and comes on with general decay of ornament all 

 over, usually after a period of compression with umbilical contrac- 

 tion. Here the catagenesis is local, and is produced in a sudden 

 jump 2; the specimen was in a flourishing state, and apparenth'- was 

 on its way (see Wright's example) to produce thicker forms. 



I have had by me for many 3''ears a considerable series of these 

 pathological specimens, having had hopes of illustrating and dis- 

 cussing them. They should be of more than ordinary interest, 

 because in connexion ,with the law of tachygenesis, and with the 

 law of catagenesis being the reversal of anagenesis, they might 

 contribute important items of knowledge to the science of 

 pathology. 



History of the figured specimen. — From a hard bluish, 

 decomposing to a soft white (chalk}') matrix of Lower Lias, out 

 of the excavation for the reservoir of the Bristol Waterworks at 

 Barrow Grurney (North Somerset) — purchased. In the aperture is 

 a ^m.2i\\ XiiDheroceras of the planicosta style. 



1 But see p. 322. 



2 It is noteworthy that, as yet, there are no kno%\ai descendants of Arietites 

 reproducing the rounded venter of this sudden iump. The nearest analogues 

 are the degenerate Asterocerates of the sagittarium pattern (see Chronological 

 paper, p. 271), and so far as is known they are evolved by gradual change and 

 normal earlier inheritance of characters (tachygenesis) as is the case with 

 other ammonites. 



