Hyatt.] 258 [April 4, 



appendage and probably also on account of its conchiolinouy struc- 

 ture easily separated from the thicker calcareous shell of the 

 apex. To close the evidence it only remains to point out the 

 close affinities of the Bactrites for Orth. pleurotomum Barr. Syst. 

 Sil. pi. 296 of Bactrites for the young of Mimoceras (Gon.) 

 compressum Beyr. Sand. Verst. Nass. pi. 11, and the straight 

 young of Agonatites (Gon.) fecundus, sp. Barr. Syst. Sil. pi. 

 11, fig. 4. 



The existence of the protoconch also removes a serious objec- 

 tion to the derivation of the Belemnoidea from the straight cones. 

 We propose to remove another by homologizing the plug on the 

 truncated cone of Orthoceras with the guard of the Belemnites. 

 We find that the central trace compares with the pseudosiphon 

 of the plug, and that the bilateral formation of the plug is similar 

 to that of the guard. This indicates to our mind, not the exist- 

 ence of two secreting organs like the arms of Argonautn, which 

 stretched back over the shell of Orthoceras, as supposed by Bar- 

 rande, but on the contrary an organ probably the homologue of 

 the dorsal fold of the mantle in Nautilus. This could readily 

 have been larger than in Nautilus and covered in the whole shell, 

 and been divided into two secreting lobes at the posterior end. 



We are thus able to account for the inclosure of the shell 

 among the Belemnoidea and the deposition of the guard, for the 

 openness of this sac as shown by Branco in the transitional form, 

 Aulacoceras of the Trias, and for its final closure as permanent 

 sac among typical Belemnites without calling to our aid any ex- 

 traordinary modifications of the known organs of Nautilus. The 

 succession here would be Orthoceras, Silurian, Aulacoceras and 

 Belemnites, Triassic. 



The Sepioidea appear to be connected with Orthoceratites 

 through Gonioceras, which resembles the broad internal shell of 

 the Sepia officinalis in the striae of growth and differs from all 

 other forms of Nautiloidea in this respect. It has also septa 

 whose outlines approximate to the outlines of the calcareous 

 layer in the interior of sepia shell, or cuttle bone. Gon. occi- 

 dentals Hall, Rep. Geo. Surv. Wiscon., 1861, p. 47, has shell and 

 septa, and the outlines of the form are also similar to Sepia, being 

 broadly fusiform, and much compressed. The loss of the proto- 

 conch can be accounted for in these forms in the same way that 

 we can account for the resorption and loss of the siphon and 



