1883.] 277 [Hyatt. 



whether nummuloidal, or tubular. Its direct connection with 

 Geisonoceras is evident, as well as the intermediate nature of 

 all its characteristics with reference to the Gomphoceraticlae. 



In this genus, as in Sactoceras, etc., the nummuloidal siphon 

 is preceded by a tubular siphon in the earlier stages of growth, 

 and this shows that we are justified in deriving this group from 

 tubular siphoned orthoceratites, and considering it as transitional 

 to the nummuloidal siphoned group of the Gomphoceratidae. 

 Sometimes the nummuloidal siphon appears to precede the tubu- 

 lar, but in the cases figured by Barrande, and in others, which we 

 have studied, the nummuloidal character was an adult peculiarity, 

 and the so called tubular character was the result of degradation 

 or shrinkage, due to old age. 



Gomphoceratidae. 

 [Equilobates. ] 



The shells have apertures with median saddles in the dorsal 

 outlines, and, therefore, an equal number of lateral lobes. 



Acleistoceras, 1 nobis, includes brevicone forms with a fusi- 

 form shape, and partially contracted living chamber. The aper- 

 ture has large ventral sinus, and a dorsal saddle, and is only 

 slightly smaller in diameter than the living chamber, and the out- 

 line is usually subtriangular. The siphon remains ventral, and 

 the form in section is an oval with the dorsum broader than the 

 venter. Silurian, Devonian, and Carboniferous. Type, Acl. (Api.) 

 olla, Saem., Paleontogr. Vol. 3, pi. 19. Mus. Comp. Zool., Cam- 

 bridge. Apiocera has been used for insects by Westwood, and the 

 original figure of Apioceras by F. de Waldh. Bull. Soc. Nat. de 

 Moscow, Yol. 17, p. 779, pi. 19, fig. 1, is not identifiable. 



G-omphoceras, Sow., Murch. Sil. Syst. Vol. 3, p. 620, 

 includes all the straight and arcuate forms, which have symmet- 

 rical T shaped apertures ; and, therefore, includes Phragmoceras, 

 and the groups Dimorion and Dimeres of M. Barrande. 



Tetrameroceras, nobis, includes Silurian species having four 

 lateral sinuses, and equals to the groups Tetramorion, and 

 Tetrameres of Barrande. Type, Tet. (Phrag.) bicinctum, sp. 

 Barr. pi. 51. 



1 AkXcuttos, open. 



