IN EXTERNAL SHELL. 



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

 next seen very distinctly, and the processes extending from the 

 head are more elongated. Here, however, I was obliged to stop, 

 this being the most perfectly developed embryo I could iind 

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

 body-whorl of the shell, where the egg-mass is attached by 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. Yoy. Samarang, 5, 1H50. 



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

 distinguish an order of cephalopoda breathing by four instead ot 

 two branchiae, and with the arms much reduced in si/e 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 pearly, and which 

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

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

 Goniatites, and with infinitely ramified 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 body of the animal, whilst the outer porcelhmous 

 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 oil 1 . This sub- 

 stance can be detected also in many fossil tetrabranchiates. 



Sandberger finds the hardness of the porcellanons layer of 

 Nautilus, K> to 5*J the nacreous layer. :;;"> to !; whilst the 

 specific gravity of the former is 2-(>(>5, and of tin- latter. l-f)!)('. 



The si nurture 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 layers of perpendicular lamina', so 

 characteristic of the hitler, being here quite indistinguishable. 

 The shell of Nautilus is the only one in which the presence of 

 tiro layers is obvious, from their difference of texture. A thin 

 iii of the external layer of the shell of X<n//tl//s ] } in i>il in.<. 

 taken parallel to the surface, shows that- it. is m;ide up of an 

 aggregation of cells of various sizes, those strata, which are 



