610 



MOLLUSCA 



PHYLUM VI 



0/ each Segment towards the interior. These internal calcareous septa are united only in 

 their peripheral parts, not meeting at the central axis so far as known, and also liahle 

 to he more or less interrupted in the transverse plane of each funnel. The interior is 

 consequently an actiniform endosiphuncle with rays extending outivardly hetween the 

 laminae of the deposits. 



Family 4, Jovellanidae Hyatt. 



Orthoceracones and cyrtoceracones ivith slightly compressed oval, or depressed and 

 more or less sub-trigonal sections. Shells smooth or partially annulated. Siphuncle large, 

 with well-developed actiniform lamellae, and distinct endosiphuncles. Apertur e open and 

 living Chamber uncontracted in the gerontic stage. 



Includes Jovellania Bayle ; and Tripleuroceras Hyatt. Silurian and Devonian. 

 Mixosiphonoceras Hyatt. Type M. (Oyrtoceras) desolatum (Barr.). Silurian and 

 Devonian. Projovellania Hyatt. Type P. {Gyrtoceras) athleta (Barr.). Silurian. 



Family 5. Rizoceratidae Hyatt. 



Orthoceracones and cyrtoceracones expanding regularly hy growth throughout life, the 

 living chamher very slightly or not sensihly contracted in the gerontic stage. Aperture con- 

 stantly open, and with slight dorsal as well as somewhat deeper and 

 hroader hyponomic sinuses. Siphuncle generally small and empty, 

 hut actiniform lamellae and an endosiphuncle sometimes occur. 

 Shells as a rule smooth or with transverse hands only, hut longitu- 

 dinal Striae are often present in earlier stages. 



Rizoceras Hyatt (Fig. 1131). Orthoceracones and exo- or 

 endogastric cyrtoceracones liaving circular or elliptical sections. 

 Living Chamber extraordinarily large and long as compared with 

 camerated part. Silurian to Carboniferous. 



Gyrtorizoceras Hyatt. Sections more com- 

 pressed than in Rizoceras, living Chamber 

 shorter and apt to be more or less laterally 

 compressed in gerontic stage, but the dorso- 

 ventral diameters only very slightly so or not 

 at all. Sutures more sinuous, and with decided 

 ventral and dorsal saddles. Type G. (Gyrtoceras) 

 minneapolis (Clarke). Ordovician and Silu- 

 rian. 



Rizoceras robustum 

 (Barr.). Silurian (E) ; 

 Bntowitz, Bohemia. 

 Aperture open. 1/2. 



Family 6. Ooceratidae Hyatt. 



Orthoceracones and gyroceracones with closely set septa and j,^if,_ 1x32. 



large nummuloidal siphuncle in later stages of the ontogeny, hut ooceras (Gyrtoceras) 



tuhular in the young. Actiniform. deposits oftener present than in bayhi (Barr.). Silurian 



,-„. ^•7 7, j. T ry 1 .77 . (E) ; Lochkow, Bohemia 



the Rizoceratidae, out not general. Jiunnels very variable, sometimes (after Barrande). 



minutely plicated or hooh-like in section, confined to dorsal side of 

 tuhe, or sometimes ahsent altogether. Living chamher short and lihe that of Gyrto- 

 rizoceras ; aperture not infrequently sub-trigonal in outline, hut alivays open. 



Ooceras Hyatt (Oonoceras Hyatt) (Fig. 1132), Cyrtoceracones more elongated 

 and usually more compressed than in Gyrtorizoceras, but otherwise similar except in 

 structure of the siphuncle. Septa rise rapidly on ventral side, and may bend sharply 

 orad, forming a funnel ridge or Shoulder on that side, but disappearing on the 



