THE DIBRANCHIATA. 463 



The Teuthidce, or Squids, are characterized by possessing 

 a pen. This is a lamellar, chitinous body, strengthened by 

 one or more longitudinal ridges, which lies in a sac lodged 

 in the anterior wall of the body, by the lining membrane of 

 which it is secreted. The posterior end of the pen is com- 

 monly broad, and its sides may be infolded so as to form a 

 conical cup (Ommastrephes). 



In the Sepiadce, or Cuttle-fishes, the sepiostaire, or " cuttle- 

 bone," which occupies the same position (Fig. 125, sh), is 

 composed of a broad plate answering to the pen, and likewise 

 infolded at its apex so as to give rise to a short cone, but cal- 

 cified. On the inner face of this plate a great number of deli- 

 cate calcified lamina?, connected by numerous short columns, 

 form a spongy tissue, which is full of air. 1 



In the Spirulidce, represented by the solitary genus JSpi- 

 rula* which is among the rarest of animals in museums, 

 though its shells are found piled up in countless millions on 

 the beaches of the islands of the Pacific, the shell is spirally 

 coiled and divided by septa, perforated by a siphuncle, into 

 chambers. The last chamber of this phragmocone, however, 

 is no larger than its predecessor, and the shell is held in posi- 

 tion by lateral processes of the mantle, which are united over 

 it, and probably represent the walls of the sac in which the 

 shell was primitively formed. The last chamber of the shell 

 lies in front of the axis of the helix ; the shell is therefore 

 coiled in the opposite direction to that of Nautilus. 



In certain extinct genera (e. g., Spirulirostra), a shell like 

 that of Spirula is inclosed in a dense and laminated pointed 

 sheath, like the hinder end of a sepiostaire, or of the pen of. 

 an Ommastrephes. 



In the Belemnitldoe (Fig. 134), which abounded in the 

 Mesozoic epoch, but have been extinct since that time, a 

 straight phragmocone is inclosed within a more or less coni- 

 cal, calcified, laminated structure, the guard or rostrum, 

 which is continued forward into a variously-shaped, usually 

 lamellar, pro-ostracum. The pro-ostracum and the rostrum 

 together represent the pen in the Teuthidce. 



The rare specimens of Belemnitidaz in which the fossil- 



1 The planes of the superimposed parallel lamince form an acute angle with 

 that of the principal plate of the sepiostaire. The connecting columns are 

 placed perpendicularly to the laminse between which they are interposed, and 

 may be simple or branched. When the young Sepia leaves the' egg, the sepi- 

 ostaire already contains air. 



2 Owen, "Zoology of the Samarang," 1848. 



