Geological Society. 579 



above the chambers is an ink-bag resting on what resembles the 

 upper part of a sepiostaire, and composed of a yellow substance 

 finely striated transversely, being formed of laminae of unequal den- 

 sity ; that in some specimens, broken longitudinally through the 

 middle, are exposed long, flat, narrow processes of a different struc- 

 ture ; that immediately beneath the superior contraction are two 

 long feather-like processes, and one or more which are short, indica- 

 ting, the author thinks, probably the situation of the mouth. With 

 reference to the first part of the paper, Mr. Pearce also notices an 

 animal allied to Sepia or Loligo, one side being covered by a pen 

 resembling that of the Loligo, and having immediately underneath 

 it, at the junction of the middle with the lower third, an ink-bag 

 resting on what resembles a sepiostaire. He mentions likewise ten or 

 twelve species of fishes, but without giving names ; also coprolites. 



2. Respecting the form of the mouth of the Ammonites and the 

 changes at different periods of growth, Mr. Pearce states his belief, 

 that the terminal lip or mouth has a different shape in the young 

 shell of almost every species, but assumes in the old a straight out- 

 line, and that he has been aware of this circumstance several years. 

 Of cases of young shells with differently shaped lips, he mentions 

 Ammonites Brongniarti (Inf. oolite), A. sublcevis (Oxf. clay), A. oh- 

 tusus (Lias), A. Kcenigii (Kelloway Rock, the mature shell is stated 

 to have a straight mouth), A. Calloviensis (Kelloway Rock, the lip of 

 the old shell is stated to be slightly contracted and to terminate with 

 gently undulating sides), A. Walcottii (Lias), and A. Goodhalli, fur- 

 nished in the mature state with a single horn-like projection at the 

 front of the mouth. In addition to these species he enumerates those 

 noticed in the preceding part of the paper. Mr. Pearce is further of 

 opinion that at different periods of the formation of the shell the la- 

 teral processes were absorbed and reproduced, and that therefore 

 they are found in various stages of growth, but are invariably want- 

 ing in the mature shell. In some species in w^hich the successive 

 mouths were much contracted or expanded, the new shell the author 

 says was continued without the absorption of the lip, leaving a highly 

 projecting rib or a deep furrow*. 



After a careful examination of upwards of twenty species in his 

 collection, with perfect mouths of all ages and from different strata, 

 not including the Oxford clay, Mr. Pearce has found the external 

 chamber to vary considerably in extent, occupying in some speci- 

 mens the whole of the last whorl, but in others less than one-third, 

 and without reference to age or species ; and he therefore suggests 

 that the young animal of the Ammonite filled the whole of the outer 

 chamber, extending also to the extreme points of the lateral pro- 

 cesses in those species which were provided with them ; and thereby 

 not only received support but afforded protection to a portion of the 

 shell extremely liable to injury. In old individuals he is of opinion 

 that the animal when quiescent was entirely contained within the 

 last chamber. 



* The author was not acquainted with M. Al. d'Orbigny's work, Pal. 

 Francalse, when he wrote the paper, and was not aware of the views given 

 in it respecting the mouth of the Ammonite. 



