THE SHELL. 33 



dora. My observations all tend to prove, as might have been 

 expected, the accuracy of Madame Power's observations on the 

 cephalopodic origin of the shell, and the fanciful nature of the 

 statements of Pliny, Poll, and the poets. 



It is quite true that the female Argonaut can readil}^ disengage 

 herself from the shell, when the velamentous arms become 

 collapsed, and float apparently useless on each side of the 

 animal ; and it is equall}^ certain that she has not the power, or, 

 more properly, the sagacity to re-enter her nest and resume the 

 guardianship of her eggs. On the contrary, she herself, if kept 

 in confinement, after darting and wounding herself against the 

 sides of the vessel in which she is confined, soon becomes 

 languid, exhausted, and very shortly dies. IS'umbers of male 

 Argonauts were taken bj^ us, at the same time, without any 

 shells, and this being the season of ovoposition may account for 

 the females, in such a number of instances, being found embracing 

 their calcareous shell-nests, which, so ingeniously formed hy the 

 instinct of the mother for the protection of her eggs from 

 injury, resemble, in some measure, those nidimental capsuled 

 secreted by many marine gastropods for the preservation of the 

 immature embryo. — Arthur Adams, Zool. Yoy. Samarang, 5, 

 1850. 



The midtilocula7- external shells, Nautilus, Ammonites, etc. 

 (iv, 63), distinguish an order of cephalopods breathing by four 

 instead of two branchiae, and with the arms much reduced in size 

 and subdivided into tentacles. The shells are capable of contain- 

 ing the entire animal in the cavity above the last aerial chamber, 

 to the wall of which it adheres 'by two strong muscles. These 

 shells are composed of two layers, the external or porcellanous 

 containing the colors, and the internal, which is pearl^^, and 

 which includes the partitions or septa. These septa, which are 

 straight or arcuated in Nautilus, in Orthoceratites, etc., are 

 angulated at the suture in Goniatites, and with infinitely ramified 

 lobes in Ammonites (xxxiv, 52), Hamites, Turrilites, and other 

 fossil genera. 



The inner pearly layer of the shell, as well as of the septa, is 

 formed by the body of the animal, whilst the outer porcellanous 

 la3^er is constructed by the mantle-margin. There is additionally 

 deposited, on the spire side of the Nautilus shell, a third thin, 

 black, grainy layer, which can be readily scraped off. This 

 substance can be detected also in many fossil tetrabranchiates. 



Sandberger finds the hardness of the porcellanous layer of 

 Nautilus, 4-5 to 5*0 ; the nacreous layer, 3'5 to 4-0 ; whilst the 

 specific gravity of the former is 2*565, and of the latter, 1-596. 



The structure of the shells of existing testaceous cephalopods 

 is, on the whole, more analogous to that of bivalves than to that 

 of the gastropods, the three layers of perpendicular laminae, so 



3 



