598 



LECTURE XXIII. 



the septa have been compared to a pile of 

 watch-glasses. They are traversed by a 

 marginal siphuncle. The septa are chiefly 

 composed of nacreous substance, with a thin 

 layer of opake and friable calcareous matter 

 on both surfaces, and the entire cone is en- 

 veloped in a sheath of opake calcareous mat- 

 ter lined with nacreous substance, and having 

 on the outer surface, or there degenerating ^ . 

 into, a pellicle or thin layer of a horny sub- 

 stance. This essential characteristic of the 

 Belemnitic shell is called the "phragmocone." 

 Its horny or albuminous tissue is continued 

 forwards beyond its base, and forms the 

 parietes of a cavity (a) containing some 

 of the viscera of the animal. In some spe- 

 cimens, two blade-shaped processes of al- 

 bumino-nacreous matter are prolonged for- 

 wards from the dorsal side of the phragmo- 

 cone. This chambered part, with its sheath, 

 is lodged in a conical cavity or alveolus* 

 excavated in the base of a conical or fusi- 

 form, sometimes compressed, usually long and 

 spathose body (c), resembling the head of a 

 dart or javelin, whence the name "Belem- 

 nite," applied to this genus of extinct Cepha- 

 lopods. This sheath or " guard " of the chambered cone is most 

 commonly found detached from the rest of the shell, with its thin 

 aveolar portion fractured, especially if fusiform, as in the younger 

 Belemnites, when it is pointed at both ends. In this state it has 

 been mistaken for the spine of an Echinoderra. 



In the typical Belemnites with a spathose guard, this consists of 

 successive layers, of which the exterior ones run parallel with the 

 outer surface, and progressively increase in length as they approach 

 it, thus forming the conical cavity for the lodgment of the chambered 

 cone. The innermost or first-formed layers of the guard are not 

 parallel with the outer ones, but recede from them at their upper 

 extremity, where they form a point, and sometimes a dilated nucleus, 

 in contact with the apex of the chambered cone. The interspace 

 thus left between the early and the later strata of the guard is occu- 

 pied by the coarse calcareous matter continued from the sheath of the 



Belemnite restored. 



• The term alveolus has been given inipropei'ly to the contents of the socket, 

 viz., to the " phragmocone." 



