TETRABRANCHIATA. 833 



the protoconch, but does not penetrate into the cavity of the latter. 

 Munier-Chalmas has shown, further, that the cavity of the proto- 

 conch is traversed by a tubular organ or " prosiphon," which abuts 

 against the front wall of the chamber, but does not communicate 

 with the proper siphuncle, the place of which it is supposed to take 

 in early life. In the A r auti/oidea, on the other hand, the initial 

 chamber of the shell (fig. 744, a, b, c) is a simple cone, which is 

 not constricted off from the first air-chamber and is not inflated. 

 The initial element of the siphuncle is a somewhat dilated caecal 

 tube, which indents the front wall of the initial chamber, but does 

 not enter its cavity. The external surface of the initial chamber is 

 usually marked by a network of transverse and longitudinal striae, 

 which mostly become obsolete in the adult shell. The hinder 

 extremity of the initial chamber also exhibits an oval, rounded, or 

 slit-like scar or cicatrix (fig. 744, a and c). The presence of this 

 cicatrix would seem to show, as held by Hyatt, that the initial 

 chamber of the Nautiloids does not correspond with the " proto- 

 conch " of the Ammonoids ; and it seems not improbable that there 

 existed in the former a deciduous protoconch, which is now repre- 

 sented only by a small vacuity in the centre of the shell (in the 

 spirally-inrolled types). Hyatt, indeed, has described and figured 

 the remains of this protoconch as occurring in certain species of 

 Orthoceras. On this view, the initial chamber of the shell of the 

 Nautiloids is really the first air-chamber, and the cicatrix in its 

 posterior wall marks the point where this communicated with the 

 caducous protoconch. 



The order of the Tetrabranchiata may be divided into the two 

 sub-orders of the Nautiloidea and Ammonoidea, typified respectively 

 by the Nautili and the Ammonites. The Ammonoidea are regarded 

 by Fischer as a separate order of the Cephalopoda ; while the char- 

 acters of the protoconch are regarded by some as affording ground 

 for a reference of this division to the Dibranchiates rather than to 

 the Tetrabranchiates. 



Regarded as a whole, the Tetrabranchiate Cephalopods form a 

 group which early attained its maximum, and is now almost extinct. 

 The Palaeozoic Tetrabranchiates are in the main of a decidedly 

 simpler type than those which followed them. The largest number 

 of generic types existed during the Mesozoic period, and the forms 

 of this epoch are morphologically the most complex. With the close 

 of the Secondary period disappeared almost all the characteristic 

 Mesozoic types, and the order was left without any representative in 

 the Tertiary rocks except the simple and ancient genus Nautilus and 

 its immediate allies. As regards the two great sections of the order, 

 the Nautiloidea are the most ancient, dating their existence from 

 the Upper Cambrian. Not only is this the case, but they are pre- 



vol. 1. 3 G 



