1883.] 283 [Hyatt. 



single row of large lateral tubercles, sutures nearly straight ; 

 siphon ventral, whorl in section depressed elliptical. Type, Ptyss. 

 fCyrt.) alienum, sp. Barr. Syst. Sil., pi. 127. 



Hercoceras, Barr. Syst. Sil., Vol. 2, Text 1, p. 152, includes 

 Silurian gyroceran forms, the type of which is Here, mirum 

 Barr.; but we also include in the same genus all the allied 

 gyroceran, and trochoceran species, like Here. (Gyr.) alatum, sp. 

 Barr. pi. 44, and Here. (Tro.) flexum, sp. Barr. pi. 44, all of which 

 have similar striae of growth, sutures, and ventral siphon. 



Anomaloceras, 1 nobis, includes but one Silurian species with 

 a nautilian shell. Anom. (Naut.) anomalus, sp. Barr. pi. 34, which 

 has a nautilian form, more involute than in Hercoceras, with 

 smooth, much depressed whorls ; siphon and sutures as in Her- 

 coceras. The one-sided position of the siphon is found also in 

 other cases among Tetrabranchiata, and is not probably a generic 

 distinction. 



Temnocheilus, McCoy, Syn. Carb. Foss. Irel. p. 20. Type, 

 T. coronatus McCoy, pi. 4, fig. 15. Cryptoceras D'Orb, Prod, de 

 Pal. Yol. 1, p. 58, has for type, Tern. (Naut.) subtuberculatus, 

 Sandb. Verst. Nass. pi. 12, fig. 3, and is a synonym. It includes 

 all the forms with smooth nautilian shells, trapezoidal whorls in 

 section, the venter very broad, the sides divergent, the dorsum 

 narrow and having always an impressed zone. A row of large 

 nodes occurs along the junction of the sides and abdomen. The 

 sutures have broad, ventral, lateral, and dorsal lobes. There are 

 no annular lobes in the Devonian forms, but they appear in some 

 Carboniferous species, as in Tem. latus, De Kon. Calc. Carb. The 

 siphon is ventral in the Devonian forms, but near the centre in 

 most of the Carboniferous species. 



Centroceras, 2 nobis, includes a series of Devonian species with 

 much compressed whorls, abdomen often hollow, sometimes nar- 

 row, with one row of tubercles along the edge of the abdomen on 

 either side. The sutures have deep V shaped ventral lobes, deep 

 lateral, and dorsal lobes ; no annular lobes in species observed. 

 The dorsum is frequently gibbous, and has an impressed zone only 

 in the more compressed, and more involute species. Cent, (Cyrt.) 



1 Avt'ji. 01 ^ ?! anomalous. 



2 Ke'vTpov, a spur. 



