258 SYNOPTIC COLLECTION. 



meeting ventrally at first, but eventually coming together 

 and joining. When this has happened, the mantle makes 

 the first layer of the guard (No. 627). Successive layers 

 are put on from without, thereby illustrating exogenous 

 growth. In many genera the guard extends a consider- 

 able distance below the chambered shell and is the part 

 most frequently preserved (see No. 628, B. subquadratus 

 Roem.). 



Thus it is seen that Belemnites carries specialization 

 so far that the shell is enclosed and a secondary struc- 

 ture, the guard, is formed. 



One of the descendants of the Belemnites is the beauti- 

 ful living Spirula (Nos. 629, 630) which has lost both the 

 pen and the guard. A perfect specimen of this animal is 

 extremely rare, 1 but the alcoholic specimen (No. 629), 

 though mutilated, shows that the shell is partly internal. 

 It also exhibits a portion of the mantle and what some 

 naturalists consider the disc of attachment. 



The shell has a large, globular protoconch finely seen 

 in No. 630. The plain concave septa are pierced by the 

 marginal siphuncle which is made up of funnels that 

 extend from one septum to another. 



Belosepia, according to Zittel, connects the Phragma- 

 phora (Belemnites, Spirula, etc.) with the Sepiophora 

 (cuttlefishes and squids). It has a guard and the 

 chambered shell is represented indistinctly. 



The young cuttlefish of to-day, Sepia officinalis Linn. 

 (PI. 631, figs, i, 2), has the interesting habit of fas- 

 tening itself, for a day or two after hatching, by a portion 

 of the lower side of the body and of the ventral arms. 

 This sucker-like area is flat and nearly colorless, and 

 reminds one forcibly of the foot of a Gastropod. Fig. i 

 is a view of the animal drawn from below when attached 

 to a glass plate ; the arms are retracted. Fig. 2 is from 



1 According to Pelseneer (Nat. Sci., VII, 1895^.63) only five 

 complete specimens are known. 



