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generalized nautiloids, Endosiphonoidea, opened into the proto- 

 conch. 



The tubular opening of the apex in Endoceras, Piloceras and 

 Actinoceras and other genera having a marked endosiphuncle, is 

 not closed by the caecum of the siphuncle as was formerly supposed. 

 It is, on the contrary, directly continuous with the endosiphuncle, 

 as was first pointed out by Foord in his Catalogue of British Cepha- 

 lopoda. This is an attenuated, central, more or less irregular tube 

 or axis formed by the extension of the points of successive endo- 

 cones or sheaths. It is more or less interrupted by pseudosepta, 

 and is a separate and distinct part occupying the axis of the large 

 siphuncle. This organ is continuous with some corresponding part 

 in the embryo which existed in the protoconch. On the other 

 hand, the true siphuncle, including the caecum of the first air cham- 

 ber, is a secondary organ formed by the funnels of the septa. The 

 living apical chamber was, as said above, a shallow cup, and its 

 limit in the living animal was probably as indicated by Henry 

 Brooks in the drawings given on PI. i of this paper. At any rate, 

 his conclusions with regard to the probable situation of the aperture 

 of this stage seem to me to be sustained by observation. 



The next substage is indicated by the presence of the caecum ly- 

 ing within the apex, and this is formed by the funnel of the first 

 septum and in association with the first septum is universal among 

 Cephalopoda, with the exception of some sepioids, so far as the in- 

 ternal structures are concerned. It has been descriptively named 

 the caecosiphonula. This may be considered as a part of the meta- 

 nepionic substage in nautiloids, but among ammonoids and belem- 

 noids it is forced back according to the law of tachygenesis into 

 the calcareous apex of the ancestral shell, being consolidated with 

 and disappearing in the aperture of the calcareous protoconch. 

 The limit of the living chamber which rested upon this first septum 

 has been determined in existing form of Nautilus pompilius by Mr. 

 Brooks and is shown in his drawings on PI. i. 



In a general way it may be also said that the external character- 

 istics of this age are characteristic of the entire order of Nauti- 

 loidea. 



Among Nautiloidea the shell of this substage grows less rapidly 

 in all its diameters and may either remain smooth and approxi- 

 mately retain the earlier form, becoming, however, more compressed, 

 or it may become more rapidly altered to a depressed ellipse. That 



