874 DIBRANCHIATE CEPHALOPODS. 



The oldest known representatives of the order appear in the Trias, 

 and belong to the extinct families of the Belemniiidce, and Belem- 

 noteuthida. The Bele?nnitidoz show a great development in the 

 Jurassic rocks, and in the same deposits occur the remains of 

 Cuttle-fishes essentially similar to those now in existence (forms of 

 the Chondrophord). The typical members of the Belemnitidce, dis- 

 appear with the close of the Cretaceous period, but a few forms of 

 this important Mesozoic family survive into the Tertiary. At the 

 present day, the sole survivor of the section of the Phragmophora is 

 the genus Spirula. The oldest types of the Sepiophora appear in 

 the Tertiary deposits. In the following are given the characters and 

 distribution in time of those families of the Dibranchiates which are 

 known to occur in the fossil condition. 



Sub-Order I. Decapoda. 



The forms included in this sub-order invariably possess an in- 

 ternal shell, and the head is always furnished with eight equal 

 "arms" and two longer "tentacles" (fig. 804, a). The mantle is 

 usually furnished with lateral or terminal fins, and the suckers of 

 the arms are pedunculate, or may be modified into horny hooks. 



The sub-order is subdivided by Fischer into the three following 

 families : — 



1. Phragmophora. — Cuttle-fishes in which the skeleton is furnished 

 with a " phragmacone," or chambered portion, in which a siphuncle is 

 developed. Ex. Spirilla, Belemnites. 



2. Sepiophora. — Cuttle-fishes in which the skeleton is calcareous, but 

 there is either no phragmacone, or a quite rudimentary one without a 

 siphuncle. Ex. Sepia. 



3. Chondrophora. — Cuttle-fishes in which the shell has the form of a 

 horny "pen." Ex. Loligo. 



Section A. Phragmophora. 



In this section the shell consists of a " phragmacone," with or 

 without certain accessory structures. The phragmacone (fig. 805) 

 resembles the shell of the Tetrabranchiates in consisting of a succes- 

 sion of air-chambers separated by shelly partitions or " septa," which 

 are perforated by a " siphuncle " ; and it more particularly resembles 

 the shell of the Ammonoids in the fact that the " protoconch " is 

 inflated and is provided with a " prosiphon." The skeleton is, how- 

 ever, internal, and possesses also other peculiarities which are not 

 found in the shell of any of the Tetrabranchiates. The only existing 

 representative of this section is the genus Spirula, but numerous 

 fossil forms are known. The Phragmophora may be divided into the 

 following three families : — 



