THE SQUID AND OCTOPUS BARTSCH. 349 



6,000 species are known. . Here form, complexity of septation, and 

 external sculpture ran riot, or, may we say, attained an overspeciali- 

 zation which soon spelled exit, for the group reached its highest 

 development in the upper Trias and disappeared suddenly and com- 

 pletely at the close of the Cretaceous. In size their shells vary from 

 the dimension of a pea to more than 6 feet in diameter. Plate 2 

 will give the reader a little more intimate view of the group. 



The third order, Belemnoidea, of the Cephalopoda, is of consider- 

 ably less antiquity, dating back only to the Triassic period with not 

 a single living representative, for the little chambered Spirula has 

 been definitely disposed among the modern 10-footed members, 

 though the paleontologists still classify it with the Belemnoidea. It 

 is among these Beiemnoids that we have to seek the ancestors of our 

 squids and cuttlefishes for, like them, they have an internal shell, but 

 of much gi-eater complexity. They also possessed the ink bag, a 

 character present in all our modern Cephalopods excepting the Nau- 

 tilus. It is quite possible that these members were as abundant in 

 these later seas as their ancestors were in their time and as their 

 descendants are to-day, but they had little of fossilizable material to 

 leave behind them at death, and thus have left a rather poor, scat- 

 tered and fragmentary record of their existence. Judging from 

 some of the pens, however, it is well to assume that the soft body 

 inclosing them may have compared favorably in size with the mem- 

 bers of the now existing fauna. Some of these pens are called fossil 

 "thunder bolts" by the iminitiated. Plate 3 shows a selection of 

 these remains. 



We next come to the modern dwellers of the seas, our " pirates 

 of the deep." In these we have either an internal skeleton or none 

 at all. In the squids the shell is embedded in the dorsal part of the 

 mantle and frequently reduced to a mere chitinoid remnant, called 

 the pen (pi. 4, fig. 1) from its resemblance to the quill pens of old. 

 At times this is decidedly reinforced by calcareous material, as shown 

 by the cuttlebone (pi. 4, fig. 2) which we are accustomed to furnish 

 our canaries, for this is the skeleton of our cuttlefish. The only coiled 

 or chambered test is found in Spirula, but here it serves not as a 

 container, but is contained within the mantle. The shell of the beauti- 

 ful Paper Nautilus or Argonaut is not a skeletal shell at all, but a 

 mere case used by the female for the protection of her eggs. 



In all these animals the body is enveloped in a soft mantle. The 

 head is strongly differentiated from the rest of the body and is sur- 

 rounded by a circle of 8 or 10 sucker-bearing arms or feet which, in 

 i-eality, are modified elements of what corresponds to the anterior 

 part of the foot in other mollusks. It is the position of these feet 

 about the head of these animals that has gained for them the name 



