MOLLUSCS. 



373 



lobsters, and crabs. For this reason the name Decacera has been substituted. Both 

 terms have reference to the number of divisions of the foot which surround tlie head, 

 the ten arms of this order contrasting strongly with the eight found in the last. 

 Besides this, there are other characters to be enumerated. The body is usually long, 

 and conies to an acute jjoint behind, and bears on either side a fin and is strengthened 

 internally by a horny ' pen ' or a calcareous ' bone.' Tlie two extra arms arise between 

 the third and fourth pairs of the Octopods, and usually differ considerably from the 

 others. Tliey are usually longer and more slender, and have the basal 

 portion narrower than tlie ape.Y. This slender portion is without 

 suckers, but the distal portions bear these organs, which, like those of 

 the other arms, are suj^j^orted on short stalks or peduncles. These 

 long arms are usually called tentacles or tentacular arms. 



Among the fossil forms I'eferred to this order, is the family known 

 as Belemxitid^, the exact relationships of which are even yet uncer- 

 tain. Different authorities have attempted to restore the anima], and 

 one of these restorations, that of Quenstedt, is shown in tlie adjacent 

 figure. Between these various restorations considerable differences 

 exist. From certain well-preserved fossils it is known that the arms 

 were furnished with hooks. The shell, however, is better preserved. 

 It consists of a pen much like that of existing forms, but to this is added 

 a calcareous chambered shell, the i)hraguiocone, the partitions of whicli 

 are traversed by a siphuncle. The apex of this phragmocone is envel- 

 oj)ed in a second calcareous shell, the rgstrum or guard. These fossils, 

 \\hich are very abundant in some parts of the Jurassic and cretaceous 

 rocks, have received a large number of popular names, indicative of 

 their general shape or of the superstitions and myths which have been 

 associated with them. Among these may be mentioned arrowheads, 

 tiiunder-bolts, petrified fingers, spectre candles, etc. In the outer 

 clianiber of the phragmocone of some specimens the remains of the ink 

 sac have been found. Some of these fossils are very large, indicatin^ 

 several feet in length. 



As we have just said, the position and structure of the Belemnites are very uncer- 

 tain. In some respects tliey seem to be related to S2yirulu ; in others, to forms like 

 Ominastrei^hes. Several attempts have been made to divide the living species of the 

 order into sub-ordinal groups, the divisions being based u])on the calcareous or horny 

 nature of tlie shell, or upon the perforate or entire condition of the cornea, but with 

 all considerable fault can be found, and the result is a highly artificial arrangement. 

 Of all the families the Spiruud.'E seems the most nearly related to the tetrabran- 

 chiatcs as well as to the Belemnites, and hence it is well to consider 

 it first. The chambered shell of Spinila at once suggests that of JVatt- 

 tilus (the only other living genus in which it is present) but, as was first 

 pointed out by Owen, the relationshi]i of tlie body to the shell is diamet- 

 rically opposite in the two ; in Ncnitiliis the ventral wall of the shell 

 describes a convex curve ; in Spirilla the reverse is the case. Another 

 important distinction exists in the fact that in the former the shell is 

 extern.al, in the latter internal ; indeed, it seems to represent the ])hrag- 

 mocone of the Belemnite, tlie guard and pen being absent, and the resemblance is 

 strengthened by an examination of the remains of the genus Spirtdirostra. In Spirilla 



Fio. 400. — Restor- 

 ation of 15elem- 

 uile. 



an animal 



Fir.. VJl. — Sj). 

 rata pevouii. 



