836 DIVISIONS OF THE CEPHALOPODA. 



they extend from septum to septum, and they may even fit into 

 one another like so many funnels (as in the genus Endoceras). 



Lastly, the " sutures" or the lines formed by the intersection 

 of the septa with the surface of the shell, are simply curved, sig- 

 moidally bent, or in some cases undulated. In a few cases short 

 external and internal bendings of the suture ("lobes") may be 

 developed, but these never constitute a conspicuous feature. 



As regards their distribution in time, the earliest types of the 

 Nautiloidea appear in the Upper Cambrian deposits, in which the 

 genera Orthoceras and Cyrtoceras are represented. In the Ordovician 

 rocks an enormous number of Nautiloids are known to occur, no 

 less than four hundred and sixty-three species having been recorded 

 by Barrande from rocks of this age in Bohemia alone. The maxi- 

 mum development of the group takes place, however, in the Silurian 

 period, the Bohemian area having yielded to the researches of Bar- 

 rande over a thousand species from deposits belonging to this 

 system. In the later Palaeozoic rocks the Nautiloids exhibit a pro- 

 gressive diminution in numbers, and only the genera Nautilus and 

 Orthoceras survive the close of this epoch, the latter finally dying 

 out in the Trias. The few known Tertiary types belong to Nautilus 

 or to closely allied forms, and the sole existing representatives of the 

 sub-order are four living species of Nautilus. 



The sub-order Nautiloidea may be divided into the following 

 families (see Foord's " Catalogue of the Fossil Cephalopoda in the 

 British Museum ") : — 



Family i. Orthoceratid^e. — In this family the shell is straight 

 or slightly curved, the aperture is simple, and the siphuncle is 

 usually slender and cylindrical. This family comprises the single 

 genus Orthoceras (figs. 747, 748), in which the shell is in the form 

 of a conical tube which is usually straight, but may be slightly 

 curved. The aperture of the shell is not contracted, and the body- 

 chamber is of large size. The septa are concave, usually horizontal, 

 and generally far apart ; and the siphuncle is usually slender and 

 cylindrical, and may be central, subcentral, or excentric in position. 

 The numerous species of Orthoceras are divided by Barrande into 

 two principal sections— the Short-coned and Long-coned forms — - 

 according as the shell has the form of a short cone with a large 

 apical angle, or of a prolonged cone with a small apical angle. The 

 first of these groups is a very small one, and almost all the more 

 common Orthocerata belong to the second section. Though the 

 shell is typically straight, it may be gently curved throughout (as in 

 O. angulatum) ; or it may be slightly bent towards the point of the 

 cone, but otherwise straight (as in O. unguis). In the smaller forms, 

 the initial chamber, with its cicatrix, is sometimes preserved. 



The oldest known species of Orthoceras appears in the Upper 



