Hyatt.] 278 [April 4, 



Hexameroceras, nobis, includes Silurian species having six 

 lateral sinuses in their apertures. Type, Hex. (Phrag.) Panderi, 

 sp. Barr. pi. 48. 



[Inequilobates.] 



This series differs from the above in having an azygos dorsal 

 sinus in place of the median dorsal saddle of the preceding genera. 

 All the genera are, so far as we know, Silurian. 



Trimeroceras, nobis, has only two lateral sinuses in addition 

 to the median sinus. Type, Tri. (Gomph.) staurostoma, sp. Barr. 

 pi. 73. 



Pentameroceras, nobis, has four lateral sinuses. Type, Pen. 

 (Gomph.) mirum, sp. Barr. pi. 82. 



Septameroceras, nobis, has six lateral sinuses. 



Trimeroceras was included by M. Barrande in his group of 

 Trimorion, and Pentameroceras in Pentamorion. Septameroceras 

 is founded upon a species in the Mus. Geol. Surv. Can., S. 

 (Gomph.) inflatum, sp. Bill. 



Mesoceratidae. 



In this group we include all those brevicones whose short, con- 

 tracted, bulbous, living chambers, and singular habit of truncat- 

 ing their shells, and general tendency to flatten the abdomino- 

 dorsal diameters of the apertures, and imperfect septa in the liv- 

 ing chambers render them very distinct as a group from all other 

 forms except the Asoceratidae. 



Mesoceras, Barr. Syst. Sil. Yol. 2, Text 5, p. 198, includes 

 but one species. This has a much flattened aperture and very 

 slight ventral sinus. It is an Acleistoceras without the vertical 

 arm in the T shaped aperture. 



Billingsites, 1 nobis, includes Silurian species having stout 

 cones, almost globular on account of their truncation and which 

 have dumb-bell shaped apertures, without ventral sinuses. Type, 

 B. (Ascoc.) Canadense, sp. Bill. Rep. Prog. Geol. Can. 1853-56, 

 p. 310, Mus. Geol. Surv. Can. This species shows that M. Bar- 

 rande is in error, in supposing that the large posterior part of the 

 living chamber can be considered as the siphon. The three last 



1 Dedicated to the memory of E. Billings. 



3 A fact already noted by Blake, British Foss. Cephalop. 



