33 2 GEOLOGICAL BIOLOGY. 



shells of straight, staff-like form (Orthoceras, Fig. 103; Baculites), slightly 

 curved (Cyrtoceras, Fig. 104), hooked (Hamites, Fig. 105), spirally enrolled 

 (Gyroceras, Fig. 106), or coiled in manner of a snail shell (Trochoceras, 

 Fig. 107). If the turns of the spirally enrolled tube are 

 in the same plane, and touch each other, the shell is 

 disk-formed (Clymenia, Trocholites, Nautilus, Ammon- 

 ites); if they turn in form of a screw, the shell is heli- 

 coidal (Cochloceras, Turrilites, Fig. 108). It is not rare 

 that the last coil is elongated in straight line, and de- 

 tached from the rest of the anterior by enrolled spiral 

 (Lituites); sometimes it is curved still more slowly in 

 the form of a hook (Ancyloceras, Macroscaphites). In 

 many of the shells spirally coiled in the same plane the 

 last turn encloses the previous turns either entirely or 

 in part. If this envelopment goes so far that the pre- 

 ceding turns are entirely concealed, and that only the 

 last one remains visible, the shell is called involute. If 

 the older coils are still visible in the centre, there is 

 then an umbilicus, and according to the degree of in- 

 volution the shell is said to have narrow or broad um- 

 bilicus. In the evolute, or open spiral, the turns do not 



t, , .... touch each other so that one can see between them. By 



FIG. 108. Tumhtes 



catenatus d'Orb. their ornamentation also the shells of Tetrabranchiata 



show considerable diversity: on the one hand there are 

 forms of which the surface is covered only by fine striae of growth, and 

 on the other are forms presenting a rich ornamentation of the surface. 

 The surface markings are smooth lines, punctate, granulate, and more 

 or less prominent lines, foliaceous excrescences, rings, protuberances, 

 simple or bifurcate ribs, tubercles, or spires, isolated or arranged in 

 series. The ornaments which follow the general direction of the longi- 

 tudinal axis of the whorl go under the name of longitudinal or spiral 

 sculpture, while those which are arranged obliquely, or at right angles 

 to these, are called transverse or radiating ornamentations. 



The position of the animal of the Nautilus (see Fig. 102) offers the only 

 good evidence by which to orient the shell of the Tetrabranchiates As 

 it turns the ventral side of the animal outwards, R. Owen has designated 

 the external or arched part of the shell, the ventral side, and the oppo- 

 site internal part, the dorsal side. All the ancient authors, who occu- 

 pied themselves exclusively with the shells, called, in the spirally en- 

 rolled forms, the external side of the shell back, and the internal side 

 the ventral side of the shell. According to Barrande, the external arched 

 part of the spirally twisted fossil forms does not always correspond to- 

 the ventral side of the animal; the convex ventral side of the shell is dis- 

 tinguished, particularly in the Nautilus, by a depression of the buccal 

 border. It is admitted, therefore, that always where such a sinus exists 

 in the buccal border it indicates the position of the siphon, and conse- 

 quently the ventral side of the animal. According to Barrande, the sinus 

 is found frequently in fossil Nautilids, sometimes upon the external 

 arched side, sometimes upon the concave inner side. There are thus, 

 evidently, exogastric and endogastric shells. In the majority of the fossil 

 shells of Cephalopoda, and particularly in the Ammonites, data are want- 



