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 wliich surround the head, 

 tlie 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 comes to an acute point 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. They are usually longer and more slender, and have the basal 

 portion narrower than the apex. This slender portion is without 

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

 the other arms, are suf)ported on short stalks or peduncles. These 

 long arms are usually called tentacles or tentacular arms. 



Among the fossil forms referred to this order, is the family known 

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

 tain. Different authorities have attempted to restore the animal, and 

 one of these restorations, that of Quenstedt, is shown in the 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 j)en much like that of existing forms, but to this is added 

 a calcareous cliambei-ed shell, the i)hragmocone, the partitions of which 

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

 oped in a second calcareous shell, the rostrum or guard. These fossils, 

 which are very abundant in some parts of the Jurassic and cretaceous 

 rocks, have received a large ninnber 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, 

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

 chamber of the phragmocone of some specimens the remains of the ink 

 sac have been found. Some of these fossils are very large, indicating an animal 

 several feet in length. 



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

 tain. In some respects they seem to be related to Spirula ; in others, to forms like 

 Ommastrephes. Several attempts have been made to divide the living species of the 

 order into sub-ordinal groups, the divisions being based upon the calcareous or horny 

 nature of the 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 Spieulid^ seems the most nearly related to the tetrabran- 

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

 it first. The chambered shell of Spirula at once suggests that of iVai«- 

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

 25ointed out by Owen, the relationship of the body to the shell is diamet- 

 rically opposite in the two ; in Nautilus the ventral wall of' the shell 

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

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

 external, in the latter internal ; indeed, it seems to represent the phrag- 

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

 strengthened by an examination of the remains of the genus Spirulirostra, In Spirula 



Fig. 490. 



Restor- 

 ation of Belem- 

 nite. 



Fic. 401. — Spir 

 riUa jjeronil. 



