CLASS V 



CEPHALOPODA 



609 



Actinoceras Bronn. (Figs. 1126-1127). Ortlioceracones and cyrtoceracones of usually 

 depressed elliptical section, with large, excessively nummuloidal siphuncle. Funnels 

 very short and crumpled, slieath almost globular. In- 

 ternal deposits contracting tbe central axis into an 

 annulated endosi])lnincle with tubuli radiating from the 

 annuli, Septa often double, with an interspace between 



the two layei's near the 



siphuncle, but solid near 



the shell. Ordovician 



to Carboniferous. 



Subgenera : Ormo- 



ceras Stokes (Fig. 1128). 



Fig. 1126. 



Actinoceras cochleatum (Schloth.)- 

 Silurian ; Gotland. Abraded frag- 

 inent showing Single septa and tliick 

 annulated endosiphuncle. 1/2. 



Fig. 1127. 



Actinoceras vertehratum Hall. 

 Silurian ; Lockport, New York. 

 Longitudinal section showing 

 organic dei)osits of siphuncle 

 (after Barrande). 



Fin. 1128. 



Actinoceras (Onnoceras) hayfieldi 

 Stokes. Ordovician ; Longitudinal 

 section showing organic deposits 

 of siphuncle partly dissolved away 

 (after Stokes). 



Ordovician to Carboniferous. Paractinoceras Hyatt (Fig. 1129). Shells longer and 

 more slender tlian in the preceding, large siphuncular segments confined to early 

 stages, and very long living Chamber. Type P. (Sactoceras) 

 canadense (Whiteaves). Silurian. 



Gyrtactinoceras Hyatt. Type C. (Cyrtoceras) rebelle (Barr.). 

 Deiroceras Hyatt; Huronia Stokes (Fig. 1130); Discosorus 

 and Gonioceras Hall. Silurian. 



(?) Tretoceras Salter. Orthocera- 

 cones having a centren nummuloidal 

 siphuncle, similar to that of Acti- 

 noceras but smaller, and with a 

 superficial tulnilar siphuncle (so- 

 called), having very long but not 

 Holocho.anoidal funnels. Sheath not 

 yet shown to be present. It is pos- 

 sible that the structure referred to 

 is a peripheral pseudo - siphuncle 

 formed by abnormal condition of the 

 septa. Silurian. 



Fi(i. 1J2; 



Actinoceras (Faractinoceras) 

 docens (Barr.) Silurian (B); 

 Dvoretz, Bohemia. Vertical 

 section showing senile stage 

 without organic deposits, 

 prcceded by adult stage with 

 siphuncular rosettes (after 

 Barrande). 



Fig. 1130. 



Huronia vertebralis 

 Stokes. Ordovician; Isle 

 Drummond, Lake Hui"on. 

 Siphuncle. 



II. AcTiNOSiPHONATA Hyatt. 



OrthoceraconeSy cyrtoceracones^ 



gyroceraconeSj and a few discoidal 



nautilicones. Shells frequently hreviconic, in which case the gerontic living Chambers and 



apertures are more highly contractecl than in all other Nautiloids. Siphuncle sometimes 



empty ; organic deposits, ivhen present 



VOL. I 



the form of laminae radiating from the sheath 



2 R 



