Hyatt.] 310 [April 4, 



sp. Barr., but the compressed forms are more numerous in the 

 Devonian than in the Silurian, like Anar. circumflexifer, and sub- 

 nautilinus Sandb. Verst. Nass. pi. 11. The closest affinites exist 

 between the young of Anar. subnautilinus, and the adults of 

 Mimoceras, and it seems probable that this genus is directly de- 

 rived from the gyrocean Mimoceratites, whether through the in- 

 termedium of Silurian or protozoic, transitional forms, we do not 

 yet know. 



Heminautilinus, nobis, includes species with whorls similar to 

 those of Anarcestes, but with angular lateral lobes in the adults. 

 The species figured by Munster as Gon. hybridus, Ueber Clym. 

 pi. 3, fig. 6, is the type. This figure enables us to see that the 

 species was a form having the sutures of Mimoceras and Anarces- 

 tes until far advanced in the adolescent stages of growth. 



Agoniatites, Meek, Paleont. Explor. 40th, Paral. Vol. 4, p. 99, 

 equals Aphyllites, Mojsis. Med. Trias Prov. p. 181. The last 

 author designates the living chambers as shorter than in Anarces- 

 tes. Agon Vanuxemi, sp. Hall, Nat. Hist. IN". Y. Vol. 5, pt. 2, pi. 59, 

 has this part of whorl fully two-thirds of a volution in length. 

 Agon, bicaniliculatus, sp. Sandb. Verst. Nass. pi. 11, in the least 

 involute variety fig. 5, has it about the same length, and in the 

 involute variety fig. 5, c-g, it is about one half of a volution. In 

 this last variety the septa are so closely set, that the funnels of 

 the siphon appear to be holochoanoidal, but close inspection 

 brings out the thin connective wall of the siphon which distin- 

 guishes the ellipochoanoidal siphon of these groups. This con- 

 nective wall is hard to demonstrate, both from its closeness to the 

 shell, and the small size of the posterior ends of the funnels. The 

 sutures have deep lateral lobes, and more prominent saddles than 

 in Anarcestes ; internally the septa are deeper than in Anarcestes, 

 and rise to a shallow concavity near the venter. The dorsal 

 lobe is to be found in the zone of impression, but is very shallow 

 has an elevated suture, and is entire, without annular lobes. 

 Comparison between Mimoceras and Anarcestes, and the larva of 

 Agon. Vanuxemi show that the dorsal lobe, as in Nautiloids, is 

 correlative with the closer coiling of the shell, and is really a su- 

 tural depression dividing the large dorsal saddles of the larva. 

 The umbilical saddles of the sutures, are formed by the increase 

 in breadth of the zone and the accompanying dorsal lobes, and in- 



