44 



PICTURESQUE SKETCHES 



14 



itself and exposes a large surface beyond the aperture 

 of the shell, and at the same time produces a slight 

 vacuum in the last chamber, its specific gravity be- 

 comes on the other hand a little less than that of wa- 

 ter, and it rises rapidly to the surface. It may also be 

 the case that the curious tube, or siphuncle, that runs 

 through all the chambers, assists in some way in 

 thus adjusting the balance of the animal ; although, 

 from the appearance of this tube, coated with a thin 

 calcareous deposit, it seems unlikely that its dilata- 

 tion or contraction could produce any useful effect.* 

 Contemporaneous with the various groups of animals 

 Figs. already described, there seem to 



have been introduced in the primae- 

 val seas a large number of species 

 very closely allied to the nautilus, 

 but provided with floats or cham- 

 bered shells not coiled into a spiral 

 as is the case with the recent ana- 

 logue, but either straight or very 

 slightly curved ; and from their re- 

 semblance to a horn, called by natu- 

 ralists Orthoceras^- (or straight horn), 

 Cyrtoceras or (bent horn), &c. The 

 animals inhabiting these shells must 

 no doubt have been very closely 

 allied to the recent nautilus ; but 

 nothing is known of them except 

 the fragments of their habitations, 

 which exhibit great variety of form 

 ORTHOCERATITES. and some rather incomprehensible 



* Owen's Lectures on Comp. Anat., p. 327 et seq. 



t Op9o (orthos), straight ; Kvproc; (cyrtus), curved ; and KtpciQ (ceras), 



horn. 



