1* EXTKKNAL SHELL. 



the 6J68 :uv large and prominent; the yolk-hag, or vitellus, is 

 next- seen very distinctly, and the processes extending from the 

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

 thN being the most perfectly devclope<l 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 by the 

 glutinous threads, are the most forward in their development . 

 while those 1 in the posterior part of the chamber are much less 

 matured. ARTHUR ADAMS, Xool. Voy. Samarang. f>, !*:><). 



The multilocular 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 sept;e. These septa-, which are M might 

 or arcuated in Nautilus, in Orthocerat iti-s. etc.. are angulated in 

 Goniatites, and with infinitely ramified lobes in Ammonites, 

 Hamites, Tiirrilites, 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 porcellanous 

 layer is constructed by the mantle-ma rgin. There is additionally 

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

 black, grainy layer, which can be readily scraped oil'. This sub- 

 stance can be detected also in many fossil let rabranchiates. 



Sandberger finds the hardness of the porcellanous layer of 

 Nautilus, K> to 5-; the nacreous layer. :;;> to !; whilst the 

 specific gravity of the former is :M'>i;5. and of the latter. 1\V.M5. 



The structure of the shells of existing testaceous cephalopods 

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

 of the n'asteropods. the three layers of perpcndicula r lamina', so 

 characteristic of the latter, being here quite indistinguishable. 

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

 fim lavers is obvious, from their difference of texture. A thin 

 -ection of the external layer of the shell of AV////////S /'o////>/7///s, 

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

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



