458 



E. H. L. Schwarz — The Apytychm. 



the other. But if the Aptychus contained gas when immersed in 

 water it would be surrounded by a film of it, and thus would be 

 rendered practically impervious to the water containing carbon 

 dioxide in solution ; while on the other hand, the shell tends to split 

 repeatedly parallel to the surface, and thus draws the water into the 

 cracks by capillary attraction, thereby exposing an enormous surface 

 to the solvent action. 



The outer layer of the Aptychus grows by simple deposition of 

 calcite on the extei'ior, obscuring often the original outer ornamenta- 

 tion ; thus in Aptychus Itevis, von Mej^er, the surface is perfectly 

 smooth, with fine punctures all over it ; but on cutting a section of 

 it radially from the umbo (Fig. 4), the primary ridges are clearly 

 seen, which would put it into the group Imbricati. Where this 

 deposit on the outer surface is thick, as in the region near the 

 umboes, the tubes communicating with the cavities of the cells of 

 the middle layer are proportionately long. No trace of communica- 

 tions through the internal layer has been observed. 



The hinge-line of the two valves shows narrow, deep cells filled 

 in with a black material ; von Meyer ^ has described them as liga- 

 ment pits, though in my preparations they do not show any opening 

 on to the exterior ; the outer boundary however, may be secondary 

 d 



Fig. 4. The same specimen as in Fig. 3, but cut radially from the umbo. 

 a, a, the primary imbricating outer layer ; b, the secondary outer 

 layer ; c, c, tubules running from the cavities of the cells to the exterior ; 

 d, d, the orifices of the tubuli plugged up with black material (carbonized 

 animal matter ?) ; e, internal layer ; /, the umbo. 



in origin, but the specimens I have cut certainly did not owe their 

 adherence to ligament, but to the tissues underlying the two halves 

 during life. 



In the group of AptycM nigrescentes the inner layer is represented 

 by a brilliant black coaly material, usually considered to be car- 

 bonized horny matter ; but there is no reason why it should not be 

 the decayed remnant of a derivative of muscle-tissue, viz. ligament. 



Whatever be the nature of the tissues forming the Aptychus, the 

 presence of tubuli reaching from the cavities of the cells to the 

 exterior is absolutely opposed to the functional internal nature of 

 the shell. 



1 Nova Acta Ac. Leop. Car. Nat. Cur., Bd. xv. No. 2, 1831 ; mentions a 

 " starkstinkende Geruch " when the Aptychus is dissolved in acid. 



