Lamarck's Genera of Shells. 77 



from one another, the last produced in a straight line. Septa 

 transverse, placed at equal distances from each other, externally 

 concave; siphon lateral, interrupted. Aperture orbicular. 



The animal of the spirula, which was brought with its shell, 

 from the South Seas by M. Peron, is a true cephalopoda. The 

 posterior part of the body is enveloped by a sac, the anterior 

 projects beyond it, and the head sustains 6ix arms, disposed like 

 a coronet, round the mouth, two of which are longer than the rest. 

 At the posterior end of the sac, is an incased shell, only a portion 

 of whose last whorl is uncovered and visible. In consequence of 

 this important discovery, Lamarck thinks himself justified in as- 

 suming that all the multilocular shells, belong to the cephalopoda. 

 One Species. Spirula Per onii*. (Nautilus Spirula, Linn.) 

 The diameter of the disc of the shell seldom exceeds an inch. 

 No further description. South Seas. PI. vi. Fig. 214. 



2. Spirolinites. 



Shell multilocular, partly twisted into a discoidal spiral; whorls 

 contiguous, the last terminating in a straight line. Septa trans- 

 verse, perforated by a tube. 



Distinguished from spirula by the contiguity of the whorls. Only 

 known in the fossil state ; very small shells ; the straight part of the 

 last whorl, bears a considerable proportion to the spiral part. Some 

 species have only an incipient spiral at the summit, the rest of the 

 shell being straight; others are quite straight, like certain indivi- 

 duals of the genus orthocera ; in some the shell is flattened, in 

 others cylindrical. In all, the septa form little external projections, 

 which divide the spiral tranversely, as by so many separate ribs or 

 stria?. The siphon, which traverses the septa and chambers, is very 

 distinct, notwithstanding the smallness of the shell. 



Type. Spirolinites cylindraceaf. 



Shell straight, curved at the apex only ; aperture orbicular. 

 Fossil, Grignon. PI. vi. Fig. 215. 2 Species. 



3. Lituolites J. 



Shell multilocular, partly twisted into a discoidal spiral ; whorls 



• Peron'i. t Cylindrical. J From lituus, a VTOoktA trumpet. 



