294 MOLLUSCA. 



in different cases. In the Calamaries the skeleton is in the 

 form of a horny " pen," consisting of a median shaft and of 

 two lateral expansions or wings. In the Sepiadce the skeleton 

 has the form of a broad, laminated, calcareous plate, having a 

 more or less perfectly chambered apex or " mucro." In the 

 singular Spirula 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 Belemnitida, 

 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 history. In addition, however, to the 

 skeleton, mandibles, and ink-bag, cases are not altogether un- 

 known in which the hooks of the suckers, and even the out- 

 lines 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 Tetrabranchiata. _ 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. Remains of the 

 living families of the Tettthidce and Sepiada are also not un- 

 known in the Mesozoic Rocks, but no trace of the great group 

 of the Belemnitida has hitherto been detected in Tertiary de- 

 posits. 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. OCTOPODA. 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. 



FAM. i. ARGONAUTIDJE. Female provided with a delicate, 

 symmetrical, involuted shell, which is secreted by the webbed 

 extremities of the two dorsal arms, and is not attached in any 

 way to the body of the animal. Male much smaller than the 

 female, shell-less. This family includes only the single genus 

 Argonauta (the Paper Nautilus). One or two species of 

 Argonaut have been discovered in the Pliocene Tertiary. 



FAM. 2. OCTOPODID^E. Shell internal, rudimentary, repre- 

 sented by two short styles encysted in the substance of the 



