1876.] 369 [Hyatt. 



growth, except in size, and the Pettos-like form is retained through- 

 oat life, except in those varieties which approximate to coronatum% 

 or more strictly speaking, except in the round abdomened varieties 

 which approximate to the predominant round abdomened forms of 

 Steph. coronatum. These last do not alter the peculiar coarse charac- 

 ter of the lateral ribs and tubercles of Blagdeni, but simply elevate 

 the abdomen and increase in size faster by growth than the normal 

 varieties, so that the umbilici become narrower, and the sides of the 

 whorls more abrupt. These are often called Aram. Banksii Sow., but 

 may be distinguished by the young which have the flat abdomen of 

 the true Blagdeni until a late period of growth, while the true 

 Banksii has young with a more elevated abdomen and larger tuber- 

 cles. The Pettos-like form of Blagdeni and its peculiar ribs are 

 more or less represented in all the young forms of the true Steph. coro- 

 natum. Sometimes specimens retain this even to an exaggerated 

 degree, growing up to the adult condition with the sides so sharp, 

 umbilici so deep, and abdomens so flat, that they appear as new 

 specific forms, until the connection is traced between them and the 

 normil forms. These are, as in the case of the similar representative 

 forms found in Steph. subcoronatum, generally rather small; such is 

 the variety known as the anceps-ornati of Quenstedt, and other scat- 

 tered varieties intermediate between this and the true broad abdo" 

 mened coronatum forms. Both Steph. subcoronatum and Blagdeni 

 occur associated in the Humphriesianus-bed, and Steph. coronatum 

 later in the Parkinsoni-bed, with the exception, perhaps, of the 

 Bank-ill variety, which may possibly occur in the Humphriesianus-bed. 

 The tendency of some varieties of the preceding species to narrow 

 the abdomen and depress the sides, is more strongly expressed in 

 Steph. coronatum than in any oiher species of this group, it having 

 become characteristic of all the adults of the normal form and of the 

 young, though in many individuals not perceptible until a late stage 

 of growth. Tuis stronger expression of an evidently inherited tend- 

 ency is accompanied by a correllative tendency to the suppression or 

 absorption of the tubercles and ribs. These changes are retrograde 

 in so far as they produce a form smaller and less ornate than the 

 preceding, and because they may be directly compared with some of 

 the retrograde changes first observed in the old age of ancestral spe- 

 cies. The tendency of the old of H umphrieslanum is to decrease the 

 size of the whorl in every way, and according to D'Orbigny, very 

 old specimens become smooth, losing tubercles and ribs. 



PROCEEDINGS C. S. 2s*. H. — VOL. XVIII. 24 DECEMBER. 187& 



