742 



ZOOLOGY 



SECT. 



pen 



phr 



which was composed of two parts, seems to have been of the 

 nature of an operculum for closing the mouth of the shell. 

 Young Ammonites, each with its aptychus, 

 have been found within the shell of the 

 parent, in which they must have remained 

 protected during their development. 



In the ordinary decapod Dibranchiata 

 the shell may consist of three parts a 

 horny pen or pro-ostraeum, a calcareous 

 guard, and a part termed the phragmocone. 

 The last, which alone represents the shell 

 of Spirula, has the form of a cone divided 

 internally by a series of septa perforated 

 by a siphuncle. These parts are most 

 completely developed in the extinct genus 

 Belemnites, in which the shell (Fig. 654) 

 consists of a straight, conical, chambered 

 phragmocone (phr.\ with a siphuncle, en- 

 closed in a calcareous sheath, the guard, 

 produced into a horny or calcareous plate, 

 the pro-ostracum (pen.). In Sepia the spine- 

 like projecting point represents the guard, 

 and the main substance of the shell is to 



be looked upon as the pro-ostracum and phragmocone, the septa 

 of the latter being represented by the calcareous lamellae. In 

 Loligo (the Squids) the shell (Fig. 647, B) is long, narrow, and 



FIG. 654. Shell of a Belem 

 nite. gd. guard ; pen, 

 pro-ostracum ; phr. phrag- 

 mocone. (From Nicholson 

 and Lydekker's PahKonto- 



Fio. C55. Shell of Argonauto argo. 



completely horny ; it corresponds to the pro-ostracum, the phrag- 

 mocone being entirely absent. 



In Octopus the shell is represented only by a pair of rudiments 

 with which muscles are connected. In Argonauta there is no shell 



