88 DIBRANCHIATE CEPHALOPODS. 



internal skeleton of the Cuttle-fishes differs very miicli in its 

 characters in different cases. In the Calaniaries the skeleton 

 is in the form of a horny " pen," consisting of a median shaft 

 and of two lateral expansions or wings (fig. 447, b). In the 

 Sepiaclce (fig. 447, a) the skeleton has the form of a broad, 

 laminated, calcareous plate, having a more or less perfectly 

 chambered apex or " mucro." In the singular Sjnrula (fig. 

 447, c and d) the skeleton has the form of a chambered tube 

 coiled into a spiral, the coils of which are separate from one 

 another. Lastly, in the extinct family of the Bdemnitidce, 

 there was a complicated internal support. It is, then, chiefly 

 from the preservation of their internal skeletons that the 

 Dibranchiate Cephalopods are known to have existed in past 

 periods of the earth's liistory. In addition, however, to the 

 skeleton and ink-bag, cases are not altoo-ether unknown in 

 which the hooks of the suckers, and even the outlines of the 

 arms and body, have been preserved in a fossil condition. 



As regards their general distribution in time, the record of 

 the Dibranchiate Cephalopods is much less complete than that 

 of the Tetrcibranchiata. In the vast series of the Palaeozoic 

 formations no trace has ever been discovered of the existence 

 of any member of this order. Shortly after the commence- 

 ment of the Mesozoic period appear the first Belemnites ; and 

 all the Secondary formations after the oldest teem with the 

 remains of this family of the Dibranchiata. Eemains of the 

 li^dng families of the Tcvthidm and Beinadm are also not 

 unknown in the Mesozoic rocks, but it is only recently that 

 any trace of the great group of the Bdemnitidce has been 

 detected in Tertiary deposits. Upon the whole, the order 

 must be regarded as having attained its maximum at the 

 present day. In the following are given the characters, 

 chief genera, and distribution in time of the families of the 

 Dibranchiate Cephalopods. 



Section A. Ogtopoda. — The Cephalopods comprised in 

 this section are distinguished by the possession of eight arms, 

 which are provided with sessile suckers. The body is short 

 and bursiform, ordinarily without fins. The shell is internal 

 and rudimentary ; in one instance only (Argonaut) external. 



YxM. 1. Argoxautid.e. — Female provided with a delicate. 



