Lamarck's Genera of Shells. 75 



externally by numerous longitudinal ribs. Chambers formed by 

 transverse septa, perforated by a central or marginal tube. 



The orthocera is a very small marine shell, resembling a straight 

 or slightly arched horn ; the subcentral siphon which traverses the 

 interior transverse septa, often projects at each extremity of the 

 shell, or only at one end. These small shells are found, with 

 many others, in the sand on the shores of the Mediterranean. 

 Type. Orthocera raphanus *. (Nautilus raphanus. Linn!) 

 Shell straight, elongated-conical, articulated ; articulations 

 torose ; siphon sublateral. Shores of the Mediterranean. PI. vi. 

 Fig. 210. 6 Species. 



3. Nodosaria f. 



Shell elongated, straight or slightly arched, subcorneal, nodular; 

 nodules globular, very smooth. Chambers formed by transverse 

 septa, perforated in the centre or near the margin. 



The nodosaria differs from the orthocera by having only smooth, 

 globular nodules on the external surface, without the small longi- 

 tudinal ribs which give the latter shell a channelled appearance. 



Type. Nodosaria radiculaX. (Nautilus radicula. Linn.) 



Shell straight, oblong-attenuated ; articulations globular, smooth ; 

 siphon sublateral. Adriatic. PI. vi. Fig. 211. 3 Species. 



4. Hippurites §. 



Shell tubular, cylindrico-conical, straight or slightly curved, 

 thick, multilocular; septa transverse. An internal lateral canal 

 formed by two longitudinal, obtuse, converging edges. The last 

 chamber closed by a thick, solid operculum ; edges of the opercu- 

 lum bevelled, and accurately adjusted to the orifice of the chamber. 



Some hippurites have a siphon which traverses the septa from 

 end to end, without communicating at all with the chambers of 

 the tube; in others, the siphon is replaced by a lateral canal some- 

 times hollow, but most commonly filled by the same septa that 

 traverse the cavity of the tube; others, again, have both the siphon 

 and canal. Those with the canal are always thicker than the 



* A 'radish. f From nodus, a knot. 



$ A little root. § From Hippuris, the herb called mare's tail? 



