.MUSCLES, ARM8 AND FINS. 25 



plates lias not been authoritatively settled however ; they have 

 been described under the names of Aptychus and Miinsteria as } 

 bivalve shells, and have also been thought to be cirripeds and 

 even the cartilages, gizzards or centrally placed cuttle-bones ! of 

 Ammonites; but the weight of opinion is decidedly in favor of 

 regarding them as opercula (first suggested by Riippell, in 1829). 

 In the Arietes group of Ammonites the operculum is a single, 

 horny, flexible piece, whilst in another group it is shell}', consist- 

 ing of two plates joined by a median suture, the exterior face 

 smooth or striated and the interior marked by growth-lines. It 

 is proper to state that Keferstein (Bronn's Klassen, iii, 1335), 

 after considering the subject at length, concludes that these were 

 not opercula, but does not give a decided opinion as to their 

 function, although admitting their connection with the Am- 

 monites. 



The outer layer of the shell has been generally destroyed in 

 fossil Ammonites, etc., leaving only the inner or nacreous and 

 more indestructible layer, which thus exhibits perfectly the edges 

 of the septa ; but in some cases it is only the outer layer that 

 has been preserved ; and frequently when the whole shell has 

 disappeared, we have perfect casts of the chambers. The de- 

 composition of the animal has contributed to form those phos- 

 phates and sulphides generally present in the body chamber, 

 whilst the permeation of water deposits crystals of calcareous 

 spar on the inner walls or sometimes even fills the entire 

 chamber. Cross-sections of fossil Ammonites with the chambers 

 filled with spar, when polished, make beautiful cabinet specimens. 

 Sometimes, as in some of the Orthocerata, the circumjacent mud 

 has invaded the air-chambers, but without entirely filling them, 

 because the contraction of the vascular lining has left empty 

 spaces between itself and a portion of the wall of each chamber. 



Muscles, Arms and Fins. 



In Nautilus we distinguish the two adductor muscles, by 

 which the animal attaches itself to the walls of its shell, and 

 which are united by a horny collar ; and within the shell Itself 

 we may notice on either side the impressions of these attach- 

 ments, sunken into the pearly walls. On the outer side of the 

 4 



