352 



MANUAL OF ZOOLOGY. 



In the family of the Sepiadce. the internal shell is cal- 

 careous (" cuttle-bone " or " sepiostaire "), and is in the 

 form of a broad plate, having an 

 imperfectly chambered apex. The 

 broad laminated plate is extremely 

 light and spongy, and the chambered 

 apex is called the " mucro." In the 

 living members of the family the body 

 is provided with long lateral fins, 

 sometimes as long and as wide as the 

 body itself. 



In the singular family of the Spirn- 

 HdcE the internal skeleton is in the form 

 of a nacreous, discoidal shell, the 

 whorls of which are not in contact 

 with one another, and which is di- 

 vided into a series of chambers by 

 means of partitions or septa which 

 are pierced by a ventral tube or 

 " siphuncle." The body is provided 

 with minute terminal fins, and the 

 arms have six rows of small suckers. 

 The shell of the Spirula commonly 

 known as the " post-horn " is similar 

 in structure to the shell of the Nau- 

 tilus, but it is lodged in the posterior 

 part of the body of the animal, and is 

 therefore internal, whereas the shell 

 of the latter is external. It really cor- 

 responds to the "phragmacone" of 

 the Belemnite. Though the shell 

 occurs in enormous numbers in cer- 

 tain localities, a single perfect speci- 

 men of the animal is all that has been 

 hitherto obtained. 



In the extinct family of the Belem- 

 nitidce, our knowledge is chiefly con- 

 fined to the hard parts. Certain 

 specimens, however, have been dis- 

 covered, which show that the Belem- 

 nite had essentially the structure of 

 a Cuttle-fish, such as the recent 

 Calamary. The body was provided with lateral fins ; the arms 

 were eight, furnished with horny hooks, with two "tentacles ; " 

 and probably the mouth was provided with horny mandibles. 



.-v 



g. 147 



nite (after Professor Phillips). 

 r Horny pen or " pro-ostra- 

 cum '," p Chambered "phrag- 

 macone" in its cavity (a) or 

 " alveolus ; " g- " Guard." 



