88o 



DIBRANCHIATE CEPHALOPODS. 



and terminates posteriorly in small pointed " mucro " (m). The 

 front portion of the mucro is hollowed out, and contains a rudimen- 

 tary " phragmacone," in which, however, a "siphuncle" is wholly 

 wanting. The pro-ostracum forms the largest portion of the shell, 

 and is thickened in front and concave on its inner side behind. 

 The convex dorsal side is formed of two strata of comparatively 

 dense calcareous tissue, partially separated by a chitinous lamella. 

 The concave inner side, on the other hand, is formed of a large 

 number of delicate parallel calcareous plates, which are not in con- 

 tact, but are united by vertical pillars, giving rise to a spongy tissue, 

 the spaces of which are filled by gases secreted by the animal. The 

 rudimentary phragmacone is imperfectly chambered, and the minute 

 "mucro" is the representative of the "guard" of the Belemnites. 



A few fossil types of Sepia are known, the oldest appearing in the 

 Eocene deposits. In the Eocene genus Belosepia the " sepion " is 

 like that of Sepia, and has a short, slightly bent rostrum. Internally 

 the rostrum is hollowed out for the reception of a chambered phrag- 

 macone, and on the inner side of this is an oblique funnel-shaped 

 cavity, which is regarded by Munier-Chalmas as representing the 

 siphuncle. Anteriorly the rostrum is continued into a calcareous 

 pro-ostracum, the dorsal surface of which is rugose. 



Section C. Chondrophora. 



In this section of the Decapods the skeleton is internal, and has 

 the form of a long, thin, pointed or feather-shaped " pen," which is 

 not calcified, or only partially so, but is composed 

 of a horn-like substance (" conchiolin "). 



This section includes a large number of the 

 commoner Cuttle-fishes of the present day, such 

 as the familiar Calamaries (Loligo, fig. 804). 

 Various fossil forms are also known from the 

 Jurassic and Cretaceous rocks, the " pens " of 

 these being often accompanied by the remains 

 of the ink-sac. The fossil "pens" consist gen- 

 erally partly of horny and partly of calcified la- 

 mellae, and some of them attain a considerable 

 size. A number of genera have been founded 

 upon these fossil pens, such as Phylloteuthis, Belo- 

 teuthis (fig. 812), Geoteuthis, Leptoteuthis, Plesio- 

 teuthis, &c. ; but the differential characters of these 

 are not of sufficient importance to require discus- 

 sion here. In some of these types, as in Plesioten- 

 this, the pen is very long and sword-shaped, and is without lateral 

 wings. More commonly, as in Beloteuthis (fig. 812), the pen is 



% 



Fig. 812. — Beloteu 

 this subcostata. Ju 

 rassic (Lias). 



