620 



MOLLUSCA 



PHYLÜM VI 



smaller azygous lobe (shown to the left in Fig. 1141, A-C) on the dorsum, 

 usually termed the 



LSm 



Suture - line of Cyrto 

 clymenkt laevigata Münst 

 Devonian. 



Suture-line of Anarcestes subnautülnus 

 (Schloth.)- Devonian. 



L EL 



Fig. 1144. 



Suture-line of Aganides sulcatus 

 (Münst.). Devonian. 



oZ2 aZl 



Left half of suture-line of Ceratites nodosus 

 de Haan. Trias. 



f/. This imdivided ventral lobe (Figs. 1143, 

 1144, EL) persists 

 throughout the Nau- 

 tilinidae so far as 

 knovvn, and is obliter- 

 ated by a secondary 

 ventral saddle only 

 in the Clymeniidae. 

 It is present throughout the ontogeny of the simplest or radical forms of 

 Glyphioceratidae, Beloceratinae and Arcestidae. But in the Devonian Gephyro- 

 ceratidae and 

 in the Triassic 

 Ceratidae, shells 

 having imdivided 

 ventral lobes have 

 notbeenrecorded; 

 in the Tirolitinae 

 but one such 



species has been doubtfully described. This class of radicals is replaced in 

 these groups by those having the ventral lobe divided by a small saddle 



usually called the ventral or 

 siphonal saddle (Fig. 1146, m). 

 The class of radicals having 

 entire ventral lobes disappears 

 before the close of the Trias. 



The entire antisiphonal lobe 

 has a more extensive distribution 

 than the entire ventral lobe, being 

 present throughout the ontogeny 

 of Nautilinidae, Clymeniidae and, 



Siphonal saddle; n, Line of inrolution. EL, Ventral (also p „„ViTTT^/^«QT.r,f I^^orv IV/Tz-wcf r^f fVio 



called siphonal or external) lobe, traversed by the siphuncle. Vrepnyroceratmae. IViOSL Ol tne 



L First or superior-laterallobe ;?, Second or i^ Glyphioceratidae have this lobe 

 lobe. ES, LS^, LS^, First second, and third lateral saddles. . , . , i •r> t • 



IS, Dorsal saddle. IL, Antisiphonal lobe, i, First dorsal entire, but it beCOmeS bind ID ths 



lobe lying on line of Involution. i , , j. • ^' i £ 



later stages oi specialised lorms. 

 The radicals of Beloceratinae have it entire, but in specialised genera it becomes 

 bifid or even trifid. It is known to be entire in only a few of the Lecanitinae, 

 and is bifid in most of the Ceratitidae and Arcestidae, besides having for the 

 most part entire sides. It is also entire in some phylogerontic species of the 

 Trias. In the Desmoceratidae and Ly toceratidae it is generally bifid, but may 

 be trifid or irregulär in some species, and is accompanied by an extraordinary 

 growth of two of the branches inwards in a large number of forms. In 

 Jurassic and Cretaceous Ammonoids, it is as a rule more or less complicated 

 by the development of secondary inflections on the sides, termed marginals. 



Paired or zygous lobes and saddles (Fig. 1141) appear between the two 

 azygous lobes and belong to two series, the laterals or externals, and the 

 dorsals. The first broad external lateral inflections, called the " first pair of 

 lateral saddles," are formed by the ventral lobe and the corresponding first 

 pair of dorsals by the formation of the antisiphonal lobe ; and between these 

 there appears a broad lobe, either wholly or the most part external (Fig. 



FiCx. 1146. 

 Kight suture-line of Coroniceras bisulcatum Brug. Lius. 



