18 EXTERNAL SHELL. 



the eyes are large and prominent ; the yolk-bag, or vitellus, is 

 next seen vei-y distinctly, and the processes extending from the 

 head are more elongated. Here, however, I was obliged to sto[), 

 this being the most perfectly developed embryo I could find 

 amongst the ova. The eggs in contact with the front part of the 

 body-whorl of the shell, where the egg-mass is attached b}- the 

 glutinous threads, are the most forward in their development, 

 while those in the posterior part of the chamber are much less 

 matured. — Arthur Adams, Zool. Voy. Samarang, 5, 1H50. 



The multilocular external shells (Nautilus, Ammonites, etc.) 

 distinguish an order of cephaloi)ods breathing by four instead ot 

 two branchia?, and with the arms much reduced in size and sub- 

 divided into tentacles. The shells are capable of containing 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 con- 

 taining the colors, and the internal, which is i)early, and which 

 includes the partitions or septje. These septie, Avhich are straight 

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

 Goniatites, and with inflnitely ramilied lobes in Ammonites, 

 Hamites, Turrilites, and other fossil genera. 



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

 formed by the bod}'^ of the animal, whilst the outer porcellanous 

 layer 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 otf. This sub- 

 stance can be detected also in many fossil tetrabranchiates. 



Sandberger finds the hardness of the porcellanous layer of 

 Nautilus, 4-5 to 5*; the nacreous layer, o-5 to 4-; whilst the 

 specific gi'avity of the former is 2-665, 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 gasteropods, the three la^^ers of perpendicular laminte, so 

 characteristic of the latter, being here quite indistinguishable. 

 The shell of Nautilus is the only one in which the ])resence of 

 two layers is obvious, from their dift'erence of texture. A thin 

 section of the external layer of the shell of Nautilus Fompilius, 

 taken parallel to the surface, shows that it is made uj) of an 

 aggregation of cells of various sizes, those strata which are 



