NAUTILID^. 



65 



where there originally existed a small fissure. Opinions 

 differ as to the function subserved by this primitive fissure ; 

 but M. Barrande thinks it possible that it transmitted a liga- 

 ment by which the young shell was temporarily attached to 

 some foreign body. The initial element of the siphuncle is 

 a somewhat dilated cpecal tube, the commencement of which 

 is attached to the front wall of the initial chamber, apparently 

 without entering into its cavity. Munier-Chalmas, however, 

 has pointed out that there exists in the interior of the initial 

 chamber an organ (" prosiphon "), which seems to take the 

 place of the siphuncle during early life. 



In the family of the Ammonitidm, on the other hand, the 

 initial portion of the shell is an inflated spheroidal, oval, or 

 pyriform body, which is termed the "ovisac" (fig. 450, d-g), 



Fig. 450.— Development of Tetrabranchiate Ceiihalopods. a, The inner end of the shell of 

 Nautilus pompilhis, enlarged, showing the initial chamber and the cicatrix ; b, Cyrtocems 

 prfeposteinim— SiliTrian, showing the commencement of the shell ; and (c) the initial chamber 

 viewed fi-om below, showing the cicatrix ; d, Inner portion of the shell of Goniatites Ucanalic- 

 tt?o«Ms— Devonian, showing the inflated ovisac ; e. First tm-n of the spire of Goniatites suh- 

 lameUosus—Devoma.n ; f, Crioceras Studeri—Cret&ceons, enlarged, showing the ovisac ; g, 

 Ovisac and first turn of the spire of Ammonites quadrisidcatus—CieUeeox-is, enlarged. (After 

 Barrande.) 



and which is separated from the first turn of the shell by a 

 distinct constriction. The ovisac communicates with the 

 first air-chamber by a narrow elliptical transverse aperture. 

 There is no structure in the ovisac which corresponds with 

 the cicatrix of the initial chamber of the Nautilidm ; and the 

 closed and dilated commencement of the siphuncle projects 

 to some distance into the cavity of the ovisac. According to 

 VOL. II. E 



